Secure a post for Margaret (3)

I believe the answer is:

peg

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

This clue could be a double definition.

'secure' is the first definition.
(thesaurus)

'a post for margaret' is the second definition.
'Peg' can be an answer for 'margaret'. I'm not certain of the 'a post for' bit.

Can you help me to learn more?

(Other definitions for peg that I've seen before include "Clothes line fastener" , "Member" , "Kind of clip for fastening clothes to line" , "Small cylindrical pin" , "The girl to hold a hat" .)

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

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