The lascivious look of a heartless lecher (4)
I believe the answer is:
leer
'the lascivious look' is the definition.
(I've seen this in another clue)
'heartless lecher' is the wordplay.
'heartless' says to hollow out the word (remove centre letters).
'lecher' with its middle taken out is 'LEER'.
'of a' acts as a link.
(Other definitions for leer that I've seen before include "Lascivious gaze" , "Give sly look" , "unpleasant look" , "Stare suggestively" , "Look lasciviously at" .)