He appears in panto, but put on weight (7)
I believe the answer is:
buttons
'he appears in panto' is the definition.
The definition suggests a singular noun but the answer is a plural noun.
'put on weight' is the wordplay.
'put' becomes 'butt' (butting is a kind of putting).
'on weight' becomes 'ons' (I can't justify this - if you can you should believe this answer much more).
'butt'+'ons'='BUTTONS'
'but' acts as a link.
Can you help me to learn more?
(Other definitions for buttons that I've seen before include "mushrooms" , "a role in pantomime" , "Secures" , "Fasteners for the Cinderella character" , "Fasteners; panto role" .)