Pole, that is, becomes a mailman (6)

I believe the answer is:

postie

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'mailman' is the definition.
(I know that mailman can be written as postie)

'pole that is' is the wordplay.
'pole' becomes 'post' (post can mean a pole or stake).
'that is' becomes 'ie' (short for Latin 'id est', 'that is').
'post'+'ie'='POSTIE'

'becomes a' acts as a link.
I am not very happy about this link. Some or all of it may be part of another bit of the clue.

Can you help me to learn more?

(Another definition for postie that I've seen is " Mail deliverer (informal)".)

I've seen this clue in the Sydney Morning Herald.

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