Matilda's mature figure (5)
I believe the answer is:
image
'figure' is the definition.
(synonyms)
'matilda's mature' is the wordplay.
'matilda's' becomes 'I'm' (where the crossword setter is 'Matilda').
'mature' becomes 'age' (synonyms).
'im'+'age'='IMAGE'
(Other definitions for image that I've seen before include "sculpture" , "Likeness of person or thing" , "Photo or likeness" , "External impression" , "Effigy, icon" .)