Hold forth about exercise function (7)
I believe the answer is:
operate
'function' is the definition.
(I know that function can be written as operate)
'hold forth about exercise' is the wordplay.
'hold forth' becomes 'orate' (I've seen this in another clue).
'about' is an insertion indicator.
'exercise' becomes 'pe' (Physical Education).
'orate' placed around 'pe' is 'OPERATE'.
(Other definitions for operate that I've seen before include "Work - perform surgery" , "Work, run" , "effect" , "Carry out surgery" , "Control, run, a mcahine say" .)