Heart sickening knowledge? (3)
I believe the answer is:
ken
'knowledge?' is the definition.
(ken is a kind of knowledge)
'heart sickening' is the wordplay.
'heart' indicates a hidden word (I've seen 'heart of' mean this).
'KEN' is hidden in the letters of 'sickening'.
(Other definitions for ken that I've seen before include "Notice" , "Awareness" , "Understanding - boy's name" , "Know; man's name" , "One's range of knowledge" .)