Don’t allow a deb to be hard-hearted (5)

I believe the answer is:

debar

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'don't allow a' is the definition.
I can't tell whether this definition defines the answer.

'deb to be hard-hearted' is the wordplay.
'to be hard' becomes 'hard' ('hard' is associated in meaning with 'hard').
'hearted' means to look at the middle letters.
The central letters of 'hard' are 'ar'.
'deb'+'ar'='DEBAR'

Can you help me to learn more?

(Other definitions for debar that I've seen before include "Formally prevent" , "Black" , "Block" , "Stop" , "Rule out" .)

I've seen this clue in the Evening Standard.

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