Man designated as CIA plant? (5)

I believe the answer is:

isaac

I'm a little stuck... Click here to teach me more about this clue!

'man' is the definition.
(I've seen this before)

'designated as cia plant?' is the wordplay.
'as cia' can be anagrammed to 'ISAAC'.
But, I am not clear how this is indicated.

Can you help me to learn more?

(Other definitions for isaac that I've seen before include "Youth almost sacrificed" , "boy almost meeting early death" , "Son almost sacrificed" , "Actor Oscar" , "Sarah's son" .)

I've seen this clue in The Globe and Mail.

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