Nearly time — time to sleep? (5)
I believe the answer is:
night
'time to sleep?' is the definition.
(I've seen this in another clue)
'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 "That's when it's darkest" , "Thing (anag)" , "See 1 Across" , "'She walks in beauty, like the . . . . .' (Byron)" , "Darkness; see you in the morning" .)