Liquid wood preservative (8)
I believe the answer is:
creosote
'liquid 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 "Wood treatment" , "Wood preserver made from coal tar" , "Oily liquid obtained from tar" , "'Dark oily liquid, preserves wood (8)'" , "Oily liquid preservative -- eco-store (anag)" .)