Round, plump and mouth-filling (7)
I believe the answer is:
orotund
'round' is the definition.
(I know that round can be written as orotund)
'plump and mouth-filling' is the wordplay.
'plump' becomes 'rotund' (I've seen this before).
'and' means one lot of letters go next to another.
'mouth' becomes 'gob' (informal term for the mouth).
'filling' indicates the central letters (the letters which fill the word).
The central letter of 'gob' is 'o'.
'rotund' after 'o' is 'OROTUND'.
(Other definitions for orotund that I've seen before include "Sonorous; pompous" , "Full, clear and musical" , "Of speech, dignified and pompous" , "Full in sound" , "Of speech, pompous and imposing" .)