Don't change this boy's headgear (7)
I believe the answer is:
stetson
'headgear' is the definition.
(Stetson is an example)
'don't change this boy's' is the wordplay.
'don't change this' becomes 'stet' (I've seen this before).
'boy' becomes 'son' (synonyms).
'stet'+'son'='STETSON'
(Other definitions for stetson that I've seen before include "St Neots (anag.)" , "capital item" , "Tip" , "Hat with broad brim and high crown" , "Hat for country-and-western singers, Texan oil magnates, etc." .)