Clue: Redcaps besiege Unionist parliament once (4)
Answer: RUMP
Explanation: Redcaps besiege Unionist parliament once (4)
**RMP** (Royal Military Police, aka "redcaps") *besiege* **U** (Unionist) 
*The original Rump Parliament was a short-lived parliament set up in 1648 after the Long Parliament was purged of those members who did not agree with charging King Charles I with high treason.