One's mature look (5)
I believe the answer is:
image
'look' is the definition.
(I've seen this in another clue)
'one's mature' is the wordplay.
'one's' becomes 'I'm' ('one is' is a formal way of saying 'I am').
'mature' becomes 'age' (synonyms).
'im'+'age'='IMAGE'
(Other definitions for image that I've seen before include "Picture; simile" , "sculpture" , "Reflection" , "External impression" , "Picture (involving one letter change from 10)" .)