Regulars leave drab wine when night is over (4)
I believe the answer is:
dawn
'when night is over' is the definition.
'regulars leave drab wine' is the wordplay.
'regulars leave' means one should take alternating letters (letters removed at regular intervals).
'drab'+'wine'='drabwine'
The alternate letters of 'drabwine' are 'DAWN'.
(Other definitions for dawn that I've seen before include "Sunrise" , "Light in East" , "Threshold" , "Cockcrow" , "beginning" .)