Leave this young fellow’s hat (7)
I believe the answer is:
stetson
'hat' is the definition.
(I know that Stetson is a type of hat)
'leave this young fellow's' is the wordplay.
'leave' becomes 'stet' (I've seen this in another clue).
'this young fellow' becomes 'son' (male child).
'stet'+'son'='STETSON'
(Other definitions for stetson that I've seen before include "Large hat" , "Capital investment" , "capital item" , "Cowboy wear" , "Hat for country-and-western singers, Texan oil magnates, etc." .)