Roadside warning (5)
I believe the answer is:
flare
'roadside warning' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for flare that I've seen before include "Widen out like a bell" , "Spread" , "Blaze; distress signal" , "(Burn with) sudden blaze" , "Blaze in feral manner" .)