He helps the priest arrange a truce (6)
I believe the answer is:
curate
'he helps the priest' is the definition.
The answer is a person as well as being a singular noun. This is suggested by the definition.
'arrange a truce' is the wordplay.
'arrange' indicates anagramming the letters.
'a'+'truce'='atruce'
'atruce' anagrammed gives 'CURATE'.
Can you help me to learn more?
(Other definitions for curate that I've seen before include "Clergyman assisting priest" , "Parish priest's assistant" , "Member of the clergy" , "assistant rector" , "Priest, a vicar's assistant" .)