Changed key before end of ballad (7)
I believe the answer is:
altered
'changed' is the definition.
('alter' can be a synonym of 'change')
'key before end of ballad' is the wordplay.
'key' becomes 'alt' (the Alt keyboard button).
'before' becomes 'ere' (poetic term for 'before').
'end of' indicates one should take the final letters.
The final letter of 'ballad' is 'd'.
'alt'+'ere'+'d'='ALTERED'
(Other definitions for altered that I've seen before include "Changed, modified" , "Change" , "Different" , "Changed, adapted" , "Treadle (anag)" .)