Speed demonstrated by Bill during exercises (4)
I believe the answer is:
pace
'speed' is the definition.
(pace is a kind of speed)
'bill during exercises' is the wordplay.
'bill' becomes 'ac' (short for account).
'during' indicates putting letters inside.
'exercises' becomes 'pe' (Physical Education).
'ac' going within 'pe' is 'PACE'.
'demonstrated by' acts as a link.
(Other definitions for pace that I've seen before include "Stride" , "Step or tempo" , "politely expressing disagreement" , "Speed of walking" , "Part of step" .)