Frown (5)
I believe the answer is:
scowl
'frown' is the definition.
(I know that scowl is a more specific form of frown)
This is all the clue.
(Other definitions for scowl that I've seen before include "Gloomy or threatening expression" , "Angry facial expression" , "Look threatening" , "Angry frown" , "Angrily frown" .)