Look lasciviously (4)
I believe the answer is:
leer
'look lasciviously' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for leer that I've seen before include "Lewd look" , "Gaze lasciviously at" , "Suggestive or sneering look" , "Look suggestively" , "Look lasciviously at" .)