Sidelong glance (4)
I believe the answer is:
leer
'sidelong glance' 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 "Gaze lasciviously at" , "Look lecherously" , "Lewd look" , "Suggestive or sneering grin" , "Give lascivious look" .)