Wood preservative (8)
I believe the answer is:
creosote
'wood preservative' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for creosote that I've seen before include "Fencing gear, perhaps" , "Preserve deal, say" , "Dark oily liquid, preservative for wood" , "Oily liquid obtained from tar" , "Oily liquid for preserving timber" .)