Plaguing of fool in poor health (9)

I believe the answer is:

assailing

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'plaguing' is the definition.
Although both the answer and definition are present participle verbs, I cannot understand how they can define each other.

'fool in poor health' is the wordplay.
'fool' becomes 'ass' (both can mean a stupid person).
'in poor health' becomes 'ailing' (an ailing person is unwell).
'ass'+'ailing'='ASSAILING'

'of' is the link.

Can you help me to learn more?

(Another definition for assailing that I've seen is " What pirates may be doing".)

I've seen this clue in The Daily Express.

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