Working out at first gym class (9)
I believe the answer is:
operating
'working' is the definition.
('operate' can be a synonym of 'work')
'out at first gym class' is the wordplay.
'out' means one lot of letters goes inside another.
'at first' becomes 'orating' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'gym class' becomes 'pe' (abbreviation for physical education).
'orating' placed around 'pe' is 'OPERATING'.
Can you help me to learn more?
(Other definitions for operating that I've seen before include "Functioning" , "Directing or controlling, machinery say" , "at work" , "(Of machine) running" , "Performing surgery" .)