Change shirt owned by real model (5)
I believe the answer is:
alter
'change' is the definition.
('alter' can be a synonym of 'change')
'shirt owned by real model' is the wordplay.
'shirt' becomes 't' (as in a T-shirt).
'owned by' indicates putting letters inside.
'model' indicates anagramming the letters.
'real' with letters rearranged gives 'aler'.
't' put into 'aler' is 'ALTER'.
(Other definitions for alter that I've seen before include "Vary" , "Change or adjust" , "convert" , "and 10 Across: Second self" , "Alert to change" .)