Crossword Genius

He sticks up for his employer (10)

Ross

I believe the answer is:

billposter

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'his employer' is the definition.
The definition suggests a singular noun which matches the answer.

'he sticks up' is the wordplay.
'he' becomes 'bill' (short for William).
'sticks up' becomes 'poster' (I can't justify this - if you can you should give a lot more credence to this answer).
'bill'+'poster'='BILLPOSTER'

'for' is the link.

Can you help me to learn more?

(Other definitions for billposter that I've seen before include "One who sticks at his job" , "One who pastes up adverts on hoardings" , "One pasting up advertisements" .)

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