Come into conflict (4,5)
(Other definitions for cross swords that I've seen before include "Have a disagreement" , "Get into a dispute" , "puzzles like this" , "fight" , "24 [WAR]" .)
'come into conflict' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for collide that I've seen before include "Dash together; clash" , "Clash; conflict" , "Hit" , "Smash together" , "Crash together" .)
'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 "Engage in struggle" , "Engage in conflict" , "Become embroiled in conflict" , "Start to do battle" , "Engage in combat" .)