Tip: solution to this clue is 'warning device' (4,5)
I believe the answer is:
tail light
'is warning device' is the definition.
(warning device on a car)
'tip solution to this clue' is the wordplay.
'tip' becomes 'tail'.
'solution to this clue' becomes 'light' (crossword answers are sometimes called 'lights').
'tail'+'light'='TAIL-LIGHT'
(Other definitions for tail light that I've seen before include "Feature of Rover, say" , "fitting on back of bike?" , "rear lamp" , "car shows it braking" , "followers'll see it" .)