Plump upon answer for building without corners (7)
I believe the answer is:
rotunda
'building without corners' is the definition.
I can't tell whether this definition defines the answer.
'plump upon answer' is the wordplay.
'plump' becomes 'rotund' (I've seen this before).
'upon' means one lot of letters go next to another.
'answer' becomes 'a' (abbreviation eg in Q and A).
'rotund'+'a'='ROTUNDA'
'for' is the link.
Can you help me to learn more?
(Other definitions for rotunda that I've seen before include "circular construction" , "Round domed building" , "Italian Pantheon, for instance" , "domed hall" , "Round domed room" .)