Egg on in parts less pure (4)
I believe the answer is:
spur
'egg on' is the definition.
(I've seen this before)
'parts less pure' is the wordplay.
'parts' says the answer is hidden in the clue (I've seen 'part of' mean this).
'SPUR' is hidden in the letters of 'less pure'.
'in' acts as a link.
(Other definitions for spur that I've seen before include "Strong incentive" , "Impetus" , "This provokes sudden movement!" , "Goad; stimulus" , "Hurry up" .)