Key before note changed (7)
I believe the answer is:
altered
'changed' is the definition.
('alter' can be a synonym of 'change')
'key before note' is the wordplay.
'key' becomes 'alt' (the Alt keyboard button).
'before' becomes 'ere' (poetic term).
'note' becomes 'D' (musical note).
'alt'+'ere'+'d'='ALTERED'
(Other definitions for altered that I've seen before include "Change" , "Reformed" , "Changed, adapted" , "Metamorphosed" , "Changed, modified" .)