Almost time — time for bed? (5)
I believe the answer is:
night
'time for bed?' is the definition.
(when people sleep)
'almost time' is the wordplay.
'almost' becomes 'nigh' (nigh can mean almost or nearly).
'time' becomes 't' (abbreviation).
'nigh'+'t'='NIGHT'
(Other definitions for night that I've seen before include "What proverbially follows day" , "Dark period" , "Sunset to sunrise" , "Darkness; see you in the morning" , "End of the day" .)