They fight with one another (6)
I believe the answer is:
allies
'they' is the definition.
Both the answer and definition are plural nouns.
Maybe you can see a link between them that I don't see?
'fight with one another' is the wordplay.
'fight' becomes 'alls' (I am not sure about this - if you are sure you should believe this answer much more).
'with' means one lot of letters goes inside another.
'one' becomes 'i' (Roman numeral).
'another' becomes 'e' (I've seen this in other clues).
'i'+'e'='ie'
'alls' placed around 'ie' is 'ALLIES'.
Can you help me to learn more?
(Other definitions for allies that I've seen before include "They join and fight together" , "Co-operative states" , "They support and encourage each other" , "Associates, confederates" , "'Friendly, helpful nations (6)'" .)