Buff partner in endless mood (7)
I believe the answer is:
amateur
'buff' is the definition.
(thesaurus)
'partner in endless mood' is the wordplay.
'partner' becomes 'mate' (synonyms).
'in' is an insertion indicator.
'endless' means to remove the last letter.
'mood' becomes 'aura' (I've seen this in another clue).
'aura' with its final letter taken off is 'aur'.
'mate' put into 'aur' is 'AMATEUR'.
(Other definitions for amateur that I've seen before include "refusing payment" , "Unpaid player" , "Of sportsman, not professional" , "Layperson" , "Unpaid (sportsman}" .)