Drink before bedtime (8)
I believe the answer is:
nightcap
'drink before bedtime' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for nightcap that I've seen before include "what last drink might be" , "Last drink before bed" , "Last drink of the day?" , "A drink before bed, wearing nocturnal headgear" , "Alcoholic drink taken at bedtime" .)