Stick the barrel inside, Tubby! (6)
I believe the answer is:
rotund
'tubby' is the definition.
(thesaurus)
'stick the barrel inside' is the wordplay.
'stick' becomes 'rod' (I've seen this before).
'the barrel' becomes 'tun' (tun is a kind of barrel).
'inside' indicates putting letters inside.
'rod' placed around 'tun' is 'ROTUND'.
(Other definitions for rotund that I've seen before include "stout" , "Pleasantly plump or rounded" , "Bulging" , "Portly" , "'Corpulent, round (6)'" .)