Dirty look (4)
I believe the answer is:
leer
'dirty look' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for leer that I've seen before include "Give sly look" , "Gaze lasciviously at" , "Give lascivious look" , "Suggestive or sneering look" , "Suggestive or sneering grin" .)