Hard-hearted line that can be deadly (5)

I believe the answer is:

arrow

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'can be deadly' is the definition.
I can't judge whether this definition defines the answer.

'hard-hearted line' is the wordplay.
'hearted' means to look at the middle letters.
'line' becomes 'row' (row is a kind of line**).
The central letters of 'hard' are 'ar'.
'ar'+'row'='ARROW'

'that' is the link.

Can you help me to learn more?

(Other definitions for arrow that I've seen before include "Pointed weapon" , "The bowman's missile" , "pointer to the way forward" , "Arm" , "This points" .)

I've seen this clue in the Evening Standard.

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