Tip: solution to this clue is 'warning device' (4,5)
I believe the answer is:
'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').
(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" .)