Tar treatment old so kept in island (8)
I believe the answer is:
creosote
'tar treatment' is the definition.
The answer and definition can be both substances as well as being singular nouns.
Perhaps they are linked in a way I don't understand?
'old so kept in island' is the wordplay.
'old' becomes 'o' (abbreviation).
'kept in' is an insertion indicator.
'island' becomes 'crete' (Greek island).
'o'+'so'='oso'
'oso' going within 'crete' is 'CREOSOTE'.
Can you help me to learn more?
(Other definitions for creosote that I've seen before include "'Dark oily liquid, preserves wood (8)'" , "Oily liquid obtained from tar" , "Dark oily liquid, preservative for wood" , "Oily liquid for preserving timber" , "Wood preserver made from coal tar" .)