Composition? Former partner let it stand outside (6)
I believe the answer is:
sextet
'composition?' is the definition.
(sextet is a kind of composition)
'former partner let it stand outside' is the wordplay.
'former partner' becomes 'ex'.
'let it stand' becomes 'stet' (I've seen this before).
'outside' is an insertion indicator.
'ex' placed into 'stet' is 'SEXTET'.
(Other definitions for sextet that I've seen before include "Group of musicians" , "A group of six singers" , "Musical group" , "Piece for six" , "Three couples" .)