Darkness close: closer in west (5)
I believe the answer is:
night
'darkness' is the definition.
(night is a period of darkness)
'close closer in west' is the wordplay.
'close' becomes 'nigh' (similar in meaning).
'closer in' suggests the final letters (I've seen 'close in' mean this).
The last letter of 'west' is 't'.
'nigh'+'t'='NIGHT'
(Other definitions for night that I've seen before include "Time of darkness" , "'The 2 8 of the dog in the _____'" , "Tennyson's "black bat"" , "Time without 10" , "''Tender is the ...'' (F Scott Fitzgerald)|" .)