More of us composed a quartet (8)
I believe the answer is:
foursome
'a quartet' is the definition.
(I know that quartet can be written as foursome)
'more of us composed' is the wordplay.
'composed' indicates anagramming the letters.
'more'+'of'+'us'='moreofus'
'moreofus' is an anagram of 'FOURSOME'.
(Other definitions for foursome that I've seen before include "In game" , "The Beatles, for example" , "Tennis doubles players, for example" , "Golfing group, perhaps" , "golf match" .)