A certain time, nearly at the end of August (5)
I believe the answer is:
night
'a certain time' is the definition.
'night' can be an answer for 'time' (night is a kind of time). I'm not sure about the 'a certain' bit.
'nearly at the end of august' is the wordplay.
'nearly' becomes 'nigh' (synonyms).
'at the end of' says to take the final letters.
The last letter of 'august' is 't'.
'nigh'+'t'='NIGHT'
Can you help me to learn more?
(Other definitions for night that I've seen before include "Time of darkness" , "death" , "Saturday -- Fever (film)" , "Darkness; see you in the morning" , "See 8" .)