Pompous, old and plump (7)
I believe the answer is:
orotund
'pompous' is the definition.
(thesaurus)
'old and plump' is the wordplay.
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'and' says to put letters next to each other.
'plump' becomes 'rotund' (I've seen this before).
'o'+'rotund'='OROTUND'
(Other definitions for orotund that I've seen before include "Resonant" , "Full in sound" , "Pompous (oratory)" , "Pompous (in style)" , "Loud or pompous" .)