Ace partner sure stripped in buff (7)
I believe the answer is:
amateur
'buff' is the definition.
(thesaurus)
'ace partner sure stripped' is the wordplay.
'ace' becomes 'a' (abbreviation).
'partner' becomes 'mate' (synonyms).
'stripped' says to take the centre (outside letters removed).
The middle letters of 'sure' are 'ur'.
'a'+'mate'+'ur'='AMATEUR'
'in' acts as a link.
(Other definitions for amateur that I've seen before include "Lay person" , "Unpaid (sportsman}" , "he's not making any money" , "One is not professional" , "Unprofessional" .)