Sour note (5)
I believe the answer is:
sharp
I believe this is a double definition.
'sour' is the first definition.
(similar in meaning)
'note' is the second definition.
(I've seen this before)
(Other definitions for sharp that I've seen before include "factor in scoring?" , "British musician" , "Incisive" , "Tangy" , "Musical notation of a half step higher than the note named" .)