Label organ note (7)
I believe the answer is:
earmark
'label' is the definition.
(thesaurus)
'organ note' is the wordplay.
'organ' becomes 'ear' (the ear is an organ).
'note' becomes 'mark' (synonyms).
'ear'+'mark'='EARMARK'
(Other definitions for earmark that I've seen before include "Designate for a specific purpose" , "Reserve specially" , "Reserve for a particular purpose" , "Set specially aside" , "Set aside for special purpose" .)