A partner with uglier exterior in buff (7)
I believe the answer is:
amateur
'buff' is the definition.
(thesaurus)
'a partner with uglier exterior' is the wordplay.
'partner' becomes 'mate' ('mate' can be a synonym of 'partner'**).
'with' means one lot of letters go next to another.
'exterior' says to hollow out the word (remove centre letters).
'uglier' with its middle removed is 'ur'.
'a'+'mate'+'ur'='AMATEUR'
'in' is the link.
(Other definitions for amateur that I've seen before include "incompetent" , "Unprofessional" , "Tum area (anag.)" , "A non-professional sportsman" , "Unpaid (sportsman}" .)