First part of a play with number going round fighting (6)
I believe the answer is:
action
'fighting' is the definition.
(I've seen this before)
'first part of a play with number going round' is the wordplay.
'first part of a play' becomes 'acti' (I've seen this in another clue).
'with' says to put letters next to each other.
'number' becomes 'no.' (abbreviation).
'going round' says the letters should be written in reverse.
'no' back-to-front is 'on'.
'acti'+'on'='ACTION'
(Other definitions for action that I've seen before include "skirmish" , "Litigation" , "Military engagement" , "The mechanism of, say, a watch" , "The process of doing something" .)