It's dark, almost, at the end of August (5)
I believe the answer is:
night
'it's dark' is the definition.
(it's dark at nighttime)
'almost at the end of august' is the wordplay.
'almost' becomes 'nigh' (nigh can mean almost or nearly).
'at the end of' suggests the final letters.
The last letter of 'august' is 't'.
'nigh'+'t'='NIGHT'
(Other definitions for night that I've seen before include "late" , "follow-up to 16 [TWILIGHT]" , "Thing (anag)" , "Darkness; see you in the morning" , "See 9" .)