They treat each other very fairly (5)
I believe the answer is:
dutch
'they treat' is the definition.
Although both the answer and definition are singular nouns, I cannot see how they can define each other.
I don't understand the remainder of the clue.
Can you help me to learn more?
(Other definitions for dutch that I've seen before include "Sort of courage" , "go fifty-fifty" , "partner" , "Better half" , "A European; pay-for-oneself (treat)" .)