Lacking a musical key (6)
I believe the answer is:
atonal
'lacking a musical key' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for atonal that I've seen before include "Unaccented" , "Keyless" , "As modernist works" , "An alto (anag) - without a fixed key" , "Not in a key (mus.)" .)