Drink before bed (8)
I believe the answer is:
nightcap
'drink before bed' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for nightcap that I've seen before include "something you get at the eleventh hour?" , "Last drink of the day?" , "A drink before bed, wearing nocturnal headgear" , "Old headgear" , "attire" .)