Fat Pole carrying cask (6)
I believe the answer is:
rotund
'fat' is the definition.
(thesaurus)
'pole carrying cask' is the wordplay.
'pole' becomes 'rod' (synonyms).
'carrying' indicates putting letters inside.
'cask' becomes 'tun' (tun is a kind of cask).
'rod' going around 'tun' is 'ROTUND'.
(Other definitions for rotund that I've seen before include "stout" , "Tubby" , "Grandiloquent; podgy" , "Large and plump" , "Pleasantly plump or rounded" .)