Don't change this boy's hat (7)
I believe the answer is:
stetson
'hat' is the definition.
(I know that Stetson is a type of hat)
'don't change this boy's' is the wordplay.
'don't change this' becomes 'stet' (instruction used in publishing).
'boy' becomes 'son' (synonyms).
'stet'+'son'='STETSON'
(Other definitions for stetson that I've seen before include "St Neots (anag.)" , "Cowboy wear" , "Hat with wide brim and high crown, maybe a cowboy's" , "Large hat" , "No tests (anag.)" .)