Beat on album captured without a key (6)
I believe the answer is:
atonal
'without a key' is the definition.
(without a musical key)
'beat on album captured' is the wordplay.
'captured' says the answer is hidden in the clue.
'ATONAL' can be found hidden inside 'beat on album'.
(Other definitions for atonal that I've seen before include "(Music) having no established key" , "(Of music) lacking key" , "Keyless" , "An alto (anag) - without a fixed key" , "(Of music) without key" .)