Autograph mail for an indicator supporter (8)

I believe the answer is:

signpost

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'an indicator supporter' is the definition.
Although both the answer and definition are singular nouns, I don't see how one could define the other.

'autograph mail' is the wordplay.
'autograph' becomes 'sign' (I've seen this in another clue).
'mail' becomes 'post' (synonyms).
'sign'+'post'='SIGNPOST'

'for' acts as a link.

Can you help me to learn more?

(Other definitions for signpost that I've seen before include "Wayside indicator" , "One may help you on your way" , "Route indicator" , "Direction indicator" , "Indication of direction" .)

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

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