Model, outsize, getting into exercise (4)
I believe the answer is:
pose
'model' is the definition.
('pose' can be a synonym of 'model')
'outsize getting into exercise' is the wordplay.
'outsize' becomes 'os'.
'getting into' indicates putting letters inside.
'exercise' becomes 'pe' (Physical Education**).
'os' going into 'pe' is 'POSE'.
(Other definitions for pose that I've seen before include "Assume an attitude (for photo etc)" , "Model; attitude" , "Attitudinise" , "Present (a question); sit" , "Pretence" .)