About to show signs of exertion, returning for a quick snooze (6)
I believe the answer is:
catnap
'quick snooze' is the definition.
(a catnap is a quick sleep)
'about to show signs of exertion returning for a' is the wordplay.
'about' becomes 'c' (abbreviation for 'circa').
'to show signs of exertion' becomes 'pant' (I've seen this in another clue).
'returning' is a reversal indicator.
'for' means one lot of letters go next to another (I've seen this in other clues).
'pant' in reverse letter order is 'tnap'.
'tnap' put after 'a' is 'atnap'.
'c'+'atnap'='CATNAP'
(Other definitions for catnap that I've seen before include "go out briefly" , "(Tom's) snooze?" , "little snooze" , "Short sleep" , "Forty winks (for moggy?)" .)