Timber preserver (3)
I believe the answer is:
tar
'timber preserver' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for tar that I've seen before include "Cigarette substance" , "Black substance, used on roads" , "Dark viscous flammable substance" , "Sticky substance - seaman" , "Smoker's black deposit" .)