Most of the alcohol knocked back? Time for bed (5)
I believe the answer is:
night
'time for bed' is the definition.
(I've seen this before)
'most of the alcohol knocked back?' is the wordplay.
'most of' means to remove the last letter.
'alcohol' becomes 'gin' (gin is a kind of alcohol).
'knocked back?' is a reversal indicator (I've seen 'knocked back' mean this).
'the' with its final letter removed is 'th'.
'th'+'gin'='thgin'
'thgin' back-to-front is 'NIGHT'.
(Other definitions for night that I've seen before include "End of the day" , "8d [DARKNESS]" , "Illumination needed now" , "Tennyson's "black bat"" , "Darkness; see you in the morning" .)