Mechanic sees speaker take exercise (8)
I believe the answer is:
operator
'mechanic' is the definition.
(I've seen this before)
'speaker take exercise' is the wordplay.
'speaker' becomes 'orator' (orator is a kind of speaker).
'take' is an insertion indicator.
'exercise' becomes 'pe' (Physical Education).
'orator' enclosing 'pe' is 'OPERATOR'.
'sees' acts as a link.
(Other definitions for operator that I've seen before include "Machinist -- or surgeon?" , "Manipulator" , "Conductor" , "Agent that handles an apparatus" , "User of machinery" .)