Fight is in balance (6)
I believe the answer is:
resist
'fight' is the definition.
(I know that resist is a type of fight)
'is in balance' is the wordplay.
'in' indicates putting letters inside.
'balance' becomes 'rest' (I've seen this before).
'is' inserted into 'rest' is 'RESIST'.
(Other definitions for resist that I've seen before include "Withstand a force" , "Withstand or defy" , "Offer opposition to" , "turn down" , "Use force to prevent something from happening to sister" .)