FBI agent’s murdered (5)

I believe the answer is:

offed

'murdered' is the definition.
Both the definition and answer are past participle verbs.
Perhaps you can see a link between them that I don't see?

'fbi agent's' is the wordplay.
'fbi' becomes 'of' (I can't explain this - if you can you should give a lot more credence to this answer).
'agent' becomes 'fed' (fed is a kind of agent).
'of'+'fed'='OFFED'

I've seen this clue in The Times.

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