Fish, yes, swimming round isle (8)
I believe the answer is:
anglesey
'isle' is the definition.
(I've seen this before)
'fish yes swimming round' is the wordplay.
'fish' becomes 'angle' (angle can mean to fish).
'swimming' indicates an anagram.
'round' indicates putting letters inside.
'yes' with letters rearranged gives 'sey'.
'angle' put into 'sey' is 'ANGLESEY'.
(Other definitions for anglesey that I've seen before include "Island off N.Wales" , "Welsh county" , "Island of North Wales" , "British island known locally as Ynys Mon" , "part of North Wales" .)