Start on introducing exercises (4)
I believe the answer is:
open
'start' is the definition.
(opening is a kind of starting)
'on introducing exercises' is the wordplay.
'introducing' means one lot of letters goes inside another (some letters introduced or inserted into others).
'exercises' becomes 'pe'.
'on' going around 'pe' is 'OPEN'.
(Other definitions for open that I've seen before include "Sports contest" , "Not shut up or hidden" , "Frank - ready for business" , "Not secret or hidden" , "begin to > crack" .)