Oppose others taking lives (6)
I believe the answer is:
resist
'oppose' is the definition.
(resisting is a kind of opposing)
'others taking lives' is the wordplay.
'others' becomes 'rest' (eg the rest of a group).
'taking' means one lot of letters goes inside another.
'lives' becomes 'is' ('be' can be a synonym of 'live').
'rest' enclosing 'is' is 'RESIST'.
(Other definitions for resist that I've seen before include "Defy for sister" , "Endure" , "Combat" , "Put up opposition, defy" , "Take stand against, oppose" .)