Clue: Attentive guard taken short (7)
Answer: MINDFUL
Explanation: Definition: Attentive

**MIND** ("guard") + **FUL**(l) ("taken", *short*)
 

 
*Full and taken as synonyms? Closest I can come up with is "all the spaces in the car park are full/taken", but I would use "have been filled" before "are full")