William and directors providing a medium for advertisements (9)

I believe the answer is:

billboard

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'providing a medium for advertisements' is the definition.
I can't judge whether this definition defines the answer.

'william and directors' is the wordplay.
'william' becomes 'bill'.
'and' is a charade indicator (letters next to each other).
'directors' becomes 'board' (board of directors).
'bill'+'board'='BILLBOARD'

Can you help me to learn more?

(Other definitions for billboard that I've seen before include "advertising space" , "Advertising hoarding" , "American hoarding" .)

I've seen this clue in The Guardian.

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