Fish certainly from the right island (8)
I believe the answer is:
anglesey
'island' is the definition.
(I know that Anglesey is a island)
'fish certainly from the right' is the wordplay.
'fish' becomes 'angle' (angle can mean to fish).
'certainly' becomes 'yes' (both can be used to answer in the affirmative).
'from the right' is a reversal indicator.
'yes' back-to-front is 'sey'.
'angle'+'sey'='ANGLESEY'
(Other definitions for anglesey that I've seen before include "Some Welsh ground" , "Island off NW tip of Wales" , "Island off N.Wales" , "part of North Wales" , "Welsh county" .)