Situation vacant; no stamp required (4-4)

I believe the answer is:

post-free

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'required' is the definition.
Both the answer and definition are adjectives. Perhaps they are linked in a way I don't understand?

'situation vacant no stamp' is the wordplay.
'situation' becomes 'post' (both can mean a job or position).
'vacant no stamp' becomes 'free' (I can't explain this - if you can you should give a lot more credence to this answer).
'post'+'free'='POST-FREE'

Can you help me to learn more?

I've seen this clue in The Irish News.

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