Knight piercing heart: that's deliberate (5)
I believe the answer is:
meant
'deliberate' is the definition.
(I've seen this before)
'knight piercing heart' is the wordplay.
'knight' becomes 'N' (abbreviation for knight in chess).
'piercing' is an insertion indicator (some letters pierce a hole in others).
'heart' becomes 'meat' (synonyms).
'n' going within 'meat' is 'MEANT'.
'that's' is the link.
(Other definitions for meant that I've seen before include "Aimed" , "planned" , "Deliberate" , "Had as intention or purpose" , "Had as purpose or design" .)