In their own fields the French are unbeatable (6)
I believe the answer is:
champs
I'm afraid I can't explain this answer.
Can you help me to learn more?
(Other definitions for champs that I've seen before include "Heaven in Paris?" , "Title holders" , "Munches noisily like horse at bit" , "frets impatiently" , "Munches noisily like abbreviated winners" .)