Fool leaves class in thought, probably having done a lot of working out (7)
I believe the answer is:
muscled
'probably having done a lot of working out' is the definition.
I can't judge whether this defines the answer.
'fool leaves class in thought' is the wordplay.
'fool leaves class' becomes 'cl' (I can't justify this - if you can you should give a lot more credence to this answer).
'in' is an insertion indicator.
'thought' becomes 'mused' (musing is a kind of thinking).
'cl' put within 'mused' is 'MUSCLED'.
Can you help me to learn more?
(Another definition for muscled that I've seen is " brawn?".)