Shield of wood old so kept in historic island (8)
I believe the answer is:
creosote
'shield of wood' is the definition.
(creosote shields or protects wood)
'old so kept in historic island' is the wordplay.
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'kept in' indicates putting letters inside.
'historic island' becomes 'Crete' (Greek island important in ancient history).
'o'+'so'='oso'
'oso' going inside 'crete' is 'CREOSOTE'.
(Other definitions for creosote that I've seen before include "Oily excretion" , "Oily liquid obtained from tar" , "Wood preservative oil" , "Fencing gear, perhaps" , "Oily, smelly wood preservative" .)