Jolly rat eschews vile treachery (6)
I believe the answer is:
cheery
'jolly' is the definition.
(thesaurus)
I don't understand the rest of the clue.
Can you help me to learn more?
(Other definitions for cheery that I've seen before include "Happy, optimistic" , "Bright - pleasant - good-humoured" , "Merry, jovial" , "Joyful -- optimistic" , "In good spirits" .)