Date: 202010
Local time of day: 0601-1200
Location: ZZZ.Airport, US
Light: Daylight
Aircraft: Commercial Fixed Wing
Operator: Air Carrier
Operating under: Part 121
Flight plan: IFR
Mission: Passenger
Flight phase: Initial Approach
Airspace: Class B ZZZ
ATC / advisory: TRACON ZZZ
Reporter organization: Air Carrier
Reporter function: Pilot Flying; First Officer
Reporter qualification: Flight Crew Multiengine; Flight Crew Air Transport Pilot (ATP)
Human factors: Time Pressure; Situational Awareness; Confusion; Communication Breakdown; Distraction
Anomaly: Deviation - Track / Heading All Types; Deviation / Discrepancy - Procedural Clearance
Detector: Person Air Traffic Control
When detected: In-flight
Result: Air Traffic Control Issued Advisory / Alert; Flight Crew FLC complied w / Automation / Advisory
Contributing factors: Human Factors; Environment - Non Weather Related
Primary problem: Human Factors

Narrative: On arrival into ZZZ; descending via on the ZZZZZ3 Arrival just before the ZZZZZ waypoint; we checked on with approach; and they responded 'Company XXXX; cleared ILS X.' I had programmed the ILS X for our arrival; but didn't select any transition on the approach. The ZZZZZ3 STAR has the note to cross ZZZZZ at 6;000 feet; expect ILS or LOC Runway X. On receipt of the clearance; I paused for a moment as there was a discontinuity in the box after ZZZZZ and before ZZZZZ1 [waypoint]; and the remaining points on the ILS X. ATC had not given us a heading or guidance for after the last fix on the STAR; just cleared ILS X. I looked at my EFB to see what the text of the STAR said; and my Captain seeing my confusion; said 'you're cleared for approach; how about direct ZZZZZ1?' He had also queued up ZZZZZ1 in the box; and was poised to execute it. I still wasn't sure; but he seemed positive. I said 'OK;' he executed it and we began direct to ZZZZZ1. Within a few seconds; Approach queried us 'I show you left of course; what [are you] doing?' The Captain replied; 'we're direct ZZZZZ1 cleared for ILS X; is there something else you would like?' ATC responded that there was a ZZZZZ transition to the ILS X; and in future that is what he meant when he cleared us ILS X - to fly the ZZZZZ transition on the ILS from the STAR. Had I looked closer at the approach plate; and not just the STAR chart; after the issuance of the clearance; the ATC clearance intention would've become obvious to me. I let my Captain cause me to second guess my hesitation; and disregard my uncertainty over our clearance. I should have requested he ask for clarification from ATC instead of allowing him to execute a path I was unsure of. I have felt a bit slow in the cockpit lately; due to having flown very infrequent this year (light schedules/hindsight/scheduled time off) and I believe I made the false assumption I was missing something and he was correct. A simple query to ATC would have cleared up the matter.I should have taken a moment longer to review the appropriate charts to clarify the appropriate flight course; and failing that answering my question; I should have queried ATC for clarification. When programming the approach; closer examination of the plate would have shown the available lead in transition from ZZZZZ. I should have trusted my hesitation upon receipt of the clearance and not assumed I was wrong in needing clarification.




Synopsis: Air Carrier First Officer reported a track deviation after missing a transition waypoint during an ILS arrival.