Forbidding scowl has expert leaving (4)
I believe the answer is:
grim
'forbidding' is the definition.
(I've seen this in another clue)
I don't understand the remainder of the clue.
Can you help me to learn more?
(Other definitions for grim that I've seen before include "Ghastly" , "Formidably stern" , "Frightful" , "Stern, cruel" , "Dreadful" .)