Note hint about surrounding darkness (5)
I believe the answer is:
night
'darkness' is the definition.
(night is a period of darkness)
'note hint about surrounding' is the wordplay.
'note' becomes 'g' (musical note).
'about' indicates anagramming the letters.
'surrounding' is an insertion indicator.
'hint' anagrammed gives 'niht'.
'g' placed into 'niht' is 'NIGHT'.
(Other definitions for night that I've seen before include "period of gloom" , "Tennyson's "black bat"" , "Thing (anag)" , "Time for turning" , "Time of darkness" .)