Old Briton OK back in island (8)
I believe the answer is:
anglesey
'island' is the definition.
(I know that Anglesey is a island)
'old briton ok back' is the wordplay.
'old briton' becomes 'Angle' (the Angles were a historical people of Britain).
'ok' becomes 'yes' (OK can mean yes).
'back' says the letters should be written in reverse.
'yes' written backwards gives 'sey'.
'angle'+'sey'='ANGLESEY'
'in' acts as a link.
(Other definitions for anglesey that I've seen before include "British island known locally as Ynys Mon" , "Island of North Wales" , "Island off NW tip of Wales" , "Some Welsh ground" , "Welsh island" .)