Legal guardian shows sign of age, wearing shirt (7)
I believe the answer is:
trustee
'legal guardian' is the definition.
(synonyms)
'sign of age wearing shirt' is the wordplay.
'sign of age' becomes 'rust' (I've seen this in another clue).
'wearing' means one lot of letters goes inside another.
'shirt' becomes 'tee shirt' (tee shirt is a kind of shirt).
'rust' going inside 'tee' is 'TRUSTEE'.
'shows' acts as a link.
(Other definitions for trustee that I've seen before include "Person who holds and administers property for another - sounds reliable" , "Person administering property for another" , "Legal administrator" , "One with powers of administration over property" , "Member of the board of, eg, a charity" .)