Note edge is forbidding (4)
I believe the answer is:
grim
'is forbidding' is the definition.
(I've seen this before)
'note edge' is the wordplay.
'note' becomes 'g' (musical note**).
'edge' becomes 'rim' (rim is a kind of edge).
'g'+'rim'='GRIM'
(Other definitions for grim that I've seen before include "Dour and severe" , "Very serious, gloomy" , "Tight-lipped and foreboding" , "Stern or severe in appearance" , "Dismal, dour" .)