Some members sat on a ledge without a key (6)
I believe the answer is:
atonal
'without a key' is the definition.
(without a musical key)
'some members sat on a ledge' is the wordplay.
'some' says the answer is hidden in the clue (some parts of the word are used).
'ATONAL' can be found hidden inside 'members sat on a ledge'.
(Other definitions for atonal that I've seen before include "(Music) having no established key" , "Keyless" , "(Of music) without key" , "Lacking traditional musical tone structure" , "Unaccented" .)