Isle with species of fish that’s not alien (4)
I believe the answer is:
mull
'isle' is the definition.
(Isle of Mull in Scotland)
'species of fish that's not alien' is the wordplay.
'species of fish' becomes 'mullet' (mullet is a type of fish).
'that's not' suggests deleting specific letters (that has not).
'alien' becomes 'et' (extra terrestrial).
'mullet' with 'et' taken out is 'MULL'.
'with' is the link.
(Other definitions for mull that I've seen before include "Inner Hebridean island" , "Warm wine; Scottish isle" , "Warm (wine) with spices; Scottish island" , "Tobermory's isle" , "Point for Scots" .)