Changed key before commencement of diminuendo (7)
I believe the answer is:
altered
'changed' is the definition.
('alter' can be a synonym of 'change')
'key before commencement of diminuendo' is the wordplay.
'key' becomes 'alt' (the Alt keyboard button).
'before' becomes 'ere' (poetic term).
'commencement of' suggests taking the first letters.
The first letter of 'diminuendo' is 'd'.
'alt'+'ere'+'d'='ALTERED'
(Other definitions for altered that I've seen before include "Different" , "Changed, adapted" , "Reformed" , "Converted" , "Metamorphosed" .)