He goes to court to protect a worker (9)
I believe the answer is:
defendant
'he goes to court' is the definition.
The answer is a person as well as being a singular noun. This is suggested by the definition.
'protect a worker' is the wordplay.
'protect' becomes 'defend' (defending is a kind of protecting).
'a worker' becomes 'ant' (worker is a type of ant).
'defend'+'ant'='DEFENDANT'
'to' is the link.
Can you help me to learn more?
(Other definitions for defendant that I've seen before include "Against whom an action is brought in a law court" , "The one before the court" , "One charged" , "Person accused" .)