A beat changed as note diminished (6)
I believe the answer is:
abated
'diminished' is the definition.
(abating is a kind of diminishing)
'a beat changed as note' is the wordplay.
'changed as' is an anagram indicator.
'note' becomes 'd' (musical note**).
'beat' anagrammed gives 'bate'.
'a'+'bate'+'d'='ABATED'
(Other definitions for abated that I've seen before include "Eased" , "Slackened off, died back" , "Calmed like storm" , "Diminished" , "dropped off" .)