Start of play about fighting (6)
I believe the answer is:
action
'fighting' is the definition.
(thesaurus)
'start of play about' is the wordplay.
'start of play' becomes 'acti' (Act I).
'about' becomes 'on'.
'acti'+'on'='ACTION'
(Other definitions for action that I've seen before include "Battle (in the law courts?)" , "instruction from director" , "claim in court" , "Combat" , "Jarndyce and Jarndyce, say" .)