When there's no actual fighting, he's great in the gym (5)
I believe the answer is:
peace
'when there's no actual fighting' is the definition.
(period without war or conflict)
'he's great in the gym' is the wordplay.
I cannot really see how this works, but
'great' could be 'ace' (I've seen this before) and 'ace' is present in the answer.
'gym' could be 'pe' (short for Physical Education) and 'pe' is found within the answer.
This accounts for all the letters.
This explanation may well be incorrect...
(Other definitions for peace that I've seen before include "Absence of conflict" , "''And I shall have some ... there, for ... comes dropping slow'' (WB Yeats)" , "[STILLNESS]" , "Accord" , "still?" .)