Wader returned after being held by Postmaster-General (5)

I believe the answer is:

egret

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'wader' is the definition.
(egret is a kind of wader)

'returned after being held by postmaster-general' is the wordplay.
'EGRET' is hidden backwards within the letters.
However, I'm not sure how this is indicated.

Can you help me to learn more?

(Other definitions for egret that I've seen before include "one with plume" , "Bird eating fish" , "Type of heron" , "(Usually) white heron" , "Type of forty eight Ac" .)

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

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