Hard to stop lust, perhaps, showing a bit of leg (4)
I believe the answer is:
shin
'a bit of leg' is the definition.
('shin' is part of 'leg')
'hard to stop lust perhaps' is the wordplay.
'hard' becomes 'h' (abbreviation used in pencil classifications).
'to stop' means one lot of letters goes inside another (stopping can mean blocking a gap).
'lust perhaps' becomes 'sin' (lust is a deadly sin).
'h' placed within 'sin' is 'SHIN'.
'showing' is the link.
(Other definitions for shin that I've seen before include "kick" , "Part of the lower leg" , "piece of beef" , "bit of leg" , "Leg-bone" .)