A quick note (5)

I believe the answer is:
sharp
This clue is a double definition.
'a quick' is the first definition.
(I've seen this in another clue)
'note' is the second definition.
(I've seen this before)
(Other definitions for sharp that I've seen before include "British musician" , "Acidic; note" , "Keen - exactly" , "needing strings slackened?" , "woman's name in Vanity Fair" .)