Fish certainly around island (8)
I believe the answer is:
anglesey
'island' is the definition.
(I've seen this before)
'fish certainly around' is the wordplay.
'fish' becomes 'angle' (synonyms).
'certainly' becomes 'yes' (I've seen this before).
'around' is a reversal indicator.
'yes' written backwards gives 'sey'.
'angle'+'sey'='ANGLESEY'
(Other definitions for anglesey that I've seen before include "Welsh county" , "Some Welsh ground" , "Island off N.Wales" , "Island of North Wales" , "part of North Wales" .)