Wood preservative, very big over in Greek island (8)
I believe the answer is:
creosote
'wood preservative' is the definition.
(I've seen this before)
'big over in greek island' is the wordplay.
'big' becomes 'os' (abbreviation for oversize).
'over' becomes 'o' (cricket abbreviation).
'in' indicates putting letters inside.
'greek island' becomes 'crete' (Crete is a Greek island).
'os'+'o'='oso'
'oso' inserted within 'crete' is 'CREOSOTE'.
'very' is the link.
(Other definitions for creosote that I've seen before include "Wood preservative oil" , "Wood-preserving oil" , "Dark oily liquid, preservative for wood" , "Oily liquid for preserving timber" , "Oily excretion" .)