Mark US agent covered around his neck (7)

I believe the answer is:

scarfed

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'covered around his neck' is the definition.
The answer and definition can be both related to bodily activities as well as being past participle verbs.
Maybe there's a link between them I don't understand?

'mark us agent' is the wordplay.
'mark' becomes 'scar' (mark left by an injury).
'us agent' becomes 'fed' (short for a federal agent).
'scar'+'fed'='SCARFED'

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