Quick setter’s becoming old in August (6)
I believe the answer is:
nimble
'quick' is the definition.
(synonyms)
'setter's becoming old in august' is the wordplay.
I cannot really see how this works, but
'setter's' could be 'im' (referring to the person who set the crossword) and 'im' is located in the answer.
This may be the basis of clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for nimble that I've seen before include "Agile, sprightly" , "'Jack be ......, Jack be quick' (6)." , "Light and quick in movement" , "Moving quickly and lightly" , "Agile - alert" .)