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' says the answer is hidden in the clue (I've seen 'heart of' mean this).
'KEN' can be found hidden inside '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" .)

I've seen this clue in the Evening Standard.

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