Calm gym expert (5)
I believe the answer is:
peace
'calm' is the definition.
(I've seen this before)
'gym expert' is the wordplay.
'gym' becomes 'pe' (short for Physical Education).
'expert' becomes 'ace' (ace is a kind of expert).
'pe'+'ace'='PEACE'
(Other definitions for peace that I've seen before include "Caught" , "Calmness" , "Accord" , "[STILLNESS]" , "Repose, tranquillity" .)