Nearly time - time for bed (5)
I believe the answer is:
night
'time for bed' is the definition.
(when people sleep)
'nearly time' is the wordplay.
'nearly' becomes 'nigh' (synonyms).
'time' becomes 't' (abbreviation).
'nigh'+'t'='NIGHT'
(Other definitions for night that I've seen before include "See 1 Across" , "From dusk until dawn" , "Darkness; see you in the morning" , "death" , "'Red sky at . . . . ., shepherd's delight'" .)