Changed key before band's final piece (7)
I believe the answer is:
altered
'changed' is the definition.
(I've seen this before)
'key before band's final piece' is the wordplay.
'key' becomes 'alt' (I've seen this in other clues**).
'before' becomes 'ere' (usually literary).
'final piece' suggests the final letters.
The final letter of 'band' is 'd'.
'alt'+'ere'+'d'='ALTERED'
(Other definitions for altered that I've seen before include "Different" , "Changed, adapted" , "Change" , "Converted" , "Treadle (anag)" .)