Decree Creole masons' patent, essentially, covers preservative substance (8)
I believe the answer is:
creosote
'decree creole masons' patent' is the definition.
Although both the answer and definition are singular nouns, I cannot see how one could define the other.
'essentially covers preservative substance' is the wordplay.
I cannot quite see how this works, but
'substance' could be 'te' (Te is an example) and 'te' is found within the answer.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for creosote that I've seen before include "Oily liquid obtained from tar" , "Oily, smelly wood preservative" , "Preserve deal, say" , "Oily liquid for preserving timber" , "Dark oily liquid, preservative for wood" .)