Sir reportedly better for a drink before bed (8)
I believe the answer is:
'a drink before bed' is the definition.
(I've seen this before)
'sir reportedly better' is the wordplay.
'sir' becomes 'knight' (knights have the title 'sir').
'reportedly' indicates a 'sounds like' (homophone) clue.
'better' becomes 'cap' (I've seen this before).
'knight' is a homophone of 'night'.
'for' acts as a link.
(Other definitions for nightcap that I've seen before include "that may go to one's head?" , "Head covering" , "Bed wear - late drink" , "Last drink before bed" , "Tipple before retiring" .)