Figure in motion, exercising (3)
I believe the answer is:
one
'figure' is the definition.
(a numerical figure)
'in motion exercising' is the wordplay.
'in' indicates a hidden word.
'ONE' is hidden within 'motion exercising'.
(Other definitions for one that I've seen before include "Single, sole" , "Single person or thing" , "Hour after midnight" , "Ace" , "Unit" .)