Car radiator additive (10)
I believe the answer is:
antifreeze
'car radiator additive' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for antifreeze that I've seen before include "which prevents ice damage" , "Winter radiator additive" , "Addition to radiator" , "Winter essential in engines" , "Liquid added to car radiator" .)