Surly note on collar (5)
I believe the answer is:
gruff
'surly' is the definition.
(I've seen this before)
'note on collar' is the wordplay.
'note' becomes 'g' (musical note).
'on' means one lot of letters go next to another (in a down clue, letters appear on others).
'collar' becomes 'ruff' (ruff is a kind of collar).
'g'+'ruff'='GRUFF'
(Other definitions for gruff that I've seen before include "Surly -- rough-spoken" , "Harsh - brusque" , "Churlish" , "Sounding rough" , "Curt and surly" .)