Nearly half of it may be black (5)
I believe the answer is:
night
'be black' is the definition.
(night can mean as black as night)
'nearly half of it' is the wordplay.
'nearly' becomes 'nigh' (synonyms).
'half of' suggests halving the letters.
'it' with half the letters taken is 't'.
'nigh'+'t'='NIGHT'
'may' is the link.
(Other definitions for night that I've seen before include "Saturday -- Fever (film)" , "Time for bed" , "Period of darkness" , "That's when it's darkest" , "Sunset to sunrise" .)