He sticks up for his employer (10)
I believe the answer is:
billposter
'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" .)