Old lover is fairly round but extremely beautiful! (9)
I believe the answer is:
exquisite
'extremely beautiful' is the definition.
(I've seen this before)
'old lover is fairly round' is the wordplay.
'old lover' becomes 'ex' (an ex-lover).
'fairly' becomes 'quite' (I've seen this before).
'round' is an insertion indicator.
'is' going within 'quite' is 'quisite'.
'ex'+'quisite'='EXQUISITE'
'but' is the link.
(Other definitions for exquisite that I've seen before include "Having special or rare beauty" , "delicate" , "Delicious" , "refined" , "Very beautiful or sensitive" .)