What one's doing is fighting (6)
I believe the answer is:
action
'is fighting' is the definition.
(I've seen this before)
'what one's doing' is the wordplay.
I cannot quite see how this works, but
'one' could be 'i' (Roman numeral) and 'i' is found in the answer.
The remaining letters 'acton' is a valid word which might be clued in a way I don't understand.
This may be the basis of clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for action that I've seen before include "Litigation" , "Accomplishment" , "Jarndyce and Jarndyce, say" , "Battle - suit" , "claim in court" .)