Note in dispute lacking content, let down (7)
I believe the answer is:
'let down' is the definition.
(I've seen this before)
'note in dispute lacking content' is the wordplay.
'note' becomes 'eflat' (E flat in music).
'in' means one lot of letters goes inside another.
'lacking content' means to remove the middle letters (the content of the word is emptied out).
'dispute' with its middle taken out is 'de'.
'eflat' put inside 'de' is 'DEFLATE'.
(Other definitions for deflate that I've seen before include "Release air from" , "Cause to shrink or collapse" , "Let air out of, like balloon" , "Puncture, collapse like balloon" , "Release gas from" .)