Come into conflict (4,5)
I believe the answer is:
lock horns
'come into conflict' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for lock horns that I've seen before include "Start to do battle" , "Engage in combat" , "join battle" , "Quarrel (like rutting stags?)" , "Come into conflict with" .)