That's quite a bit of fighting for a cross (6)
I believe the answer is:
thwart
'cross' is the definition.
(I know this)
I can't explain the rest of the clue.
Can you help me to learn more?
(Other definitions for thwart that I've seen before include "Frustrate or foil" , "seat for oarsman" , "Hinder or prevent" , "check" , "Oppose successfully" .)