Getting into flat on a ladder, without a key! (6)
I believe the answer is:
atonal
'without a key' is the definition.
(without a musical key)
'getting into flat on a ladder' is the wordplay.
'getting into' says the answer is hidden in the clue.
'ATONAL' is hidden in the letters of 'flat on a ladder'.
(Other definitions for atonal that I've seen before include "Musically, having no established key" , "Lacking traditional musical tone structure" , "Unaccented" , "Not in a key (mus.)" , "An alto (anag) - without a fixed key" .)