Warning device in flight, say, is to get engaged in conflict (4,5)
I believe the answer is:
lock horns
'is to get engaged in conflict' is the definition.
'warning device in flight' is the wordplay.
'warning device' becomes 'horn' (horn is a kind of warning device).
'in' is an insertion indicator.
'flight' becomes 'locks' (a flight is a sequence of canal locks).
'horn' going inside 'locks' is 'LOCK HORNS'.
'say' is the link.
(Other definitions for lock horns that I've seen before include "Start to do battle" , "Engage in struggle" , "Engage in combat" , "Quarrel (like rutting stags?)" , "join battle" .)