Combat with lives in balance (6)
I believe the answer is:
resist
'combat' is the definition.
(I've seen this before)
'lives in balance' is the wordplay.
'lives' becomes 'is' ('be' can be a synonym of 'live').
'in' indicates putting letters inside.
'balance' becomes 'rest' (I've seen this before).
'is' inserted within 'rest' is 'RESIST'.
'with' is the link.
(Other definitions for resist that I've seen before include "Refuse to give in to" , "Withstand, oppose" , "Put up fight, oppose forcefully" , "Combat" , "Withstand or defy" .)