Agent (ailing) secreting warrant (6)

I believe the answer is:

entail

'warrant' is the definition.
(entail can mean to warrant or involve)

'agent ailing secreting' is the wordplay.
'secreting' says the answer is hidden in the clue.
'ENTAIL' is hidden in the letters of 'agent ailing'.

(Other definitions for entail that I've seen before include "If willing, this settles" , "Invlove or necessitate" , "Involve or necessitate" , "Call for" , "Involve or make necessary" .)

I've seen this clue in The Telegraph.

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