A note that spells trouble (3)
I believe the answer is:
ado
'trouble' is the definition.
(both can mean bother or commotion)
'a note' is the wordplay.
'note' becomes 'do' (musical note, as in do-re-mi).
'a'+'do'='ADO'
'that spells' is the link.
(Other definitions for ado that I've seen before include "Fuss or bustle" , "agitation" , "stir" , "Shakespeare: 'Much ... About Nothing'(3) ()" , "Fuss, difficulty" .)