Criticise a fool for some failing (6)

I believe the answer is:

assail

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'criticise' is the definition.
(I know that assail is a more specific form of criticise)

'a fool for some failing' is the wordplay.
'a fool for' becomes 'assai' (I can't justify this - if you can you should give a lot more credence to this answer).
'some' indicates the central letters.
The middle of 'failing' is 'l'.
'assai'+'l'='ASSAIL'

Can you help me to learn more?

(Other definitions for assail that I've seen before include "Criticise severely" , "Sudden attack" , "Aggressively attack" , "Charge" , "Criticize" .)

I've seen this clue in the Evening Standard.

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