Stick round beer cask, spherical (6)
I believe the answer is:
rotund
'spherical' is the definition.
(I've seen this before)
'stick round beer cask' is the wordplay.
'stick' becomes 'rod' (I've seen this before).
'round' is an insertion indicator.
'beer cask' becomes 'tun' (I've seen this before).
'rod' placed around 'tun' is 'ROTUND'.
(Other definitions for rotund that I've seen before include "Pleasantly plump or rounded" , "stout" , "Portly" , "Tubby" , "Large and plump" .)