Struggle is in balance (6)
I believe the answer is:
resist
'struggle' is the definition.
(I know that resist is a type of defend)
'is in balance' is the wordplay.
'in' is an insertion indicator.
'balance' becomes 'rest' (I've seen this before).
'is' put into 'rest' is 'RESIST'.
(Other definitions for resist that I've seen before include "Combat" , "Hold out for sister" , "Put up opposition to sister" , "Withstand a force" , "'Withstand, fight against (6)'" .)