Instrument that usually provides a warning to drivers (8)
I believe the answer is:
triangle
This clue is a double definition.
'instrument' is the first definition.
(triangle is a kind of instrument)
'that usually provides a warning to drivers' is the second definition.
(UK road signs with warnings are usually triangular)
(Other definitions for triangle that I've seen before include "Simple percussion instrument" , "Large tin (anag.)" , "Percussion instrument of geometric origin" , "Regular shape with" , "Three-sided figure (maths)" .)