Restrict online messages — and entrance supporters? (9)

I believe the answer is:

gateposts

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'entrance supporters?' is the definition.
Although both the answer and definition are plural nouns, I cannot understand how they can define each other.

'restrict online messages' is the wordplay.
'restrict' becomes 'gate' (gating is a kind of restricting).
'online messages' becomes 'posts' (I can't explain this - if you can you should believe this answer much more).
'gate'+'posts'='GATEPOSTS'

'and' is the link.

Can you help me to learn more?

I've seen this clue in The Times.

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