To round off, become pompous (7)
I believe the answer is:
orotund
'pompous' is the definition.
(I've seen this in another clue)
'to round off' is the wordplay.
'off' indicates an anagram.
'to'+'round'='toround'
'toround' with letters rearranged gives 'OROTUND'.
'become' is the link.
(Other definitions for orotund that I've seen before include "Full, clear and musical" , "Resonant" , "Not dour (anag.)" , "Full, round" , "booming" .)