Mudguard (6)
I believe the answer is:
fender
'mudguard' is the definition.
(synonyms)
This is all the clue.
(Other definitions for fender that I've seen before include "Fire- or wheel-guard" , "Feature of vehicle from 24 [US SAY]" , "Boat's impact protector" , "Hearth-guard" , "On a boat, it's protects against impact" .)