Exposed wood treated with secret preservative (8)
I believe the answer is:
creosote
'preservative' is the definition.
(I've seen this before)
'exposed wood treated with secret' is the wordplay.
'exposed' says to take the centre (outside letters removed).
'treated with' is an anagram indicator.
The central letters of 'wood' are 'oo'.
'oo' mixed up with 'secret' makes 'CREOSOTE'.
(Other definitions for creosote that I've seen before include "Wood-preserving oil" , "Oily excretion" , "Oily, smelly wood preservative" , "Dark oily liquid, preservative for wood" , "Oily liquid obtained from tar" .)