This is meant to arrest one returning from Mediterranean island (8)
I believe the answer is:
sicilian
'this is meant to arrest one returning from mediterranean' is the definition.
The definition suggests a singular noun which matches the answer.
'island' is the wordplay.
I cannot really see how this works, but
'island' could be 'sicilia' (Sicilia is an example) and 'sicilia' is present in the answer.
A single letter 'n' remains which might be clued in a way I don't understand.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for sicilian that I've seen before include "One from Palermo, say" , "of Mafia origin" , "Mafioso may be" , "Native or large island off the toe of Italy" , "Inhabitant of Island with Mount Etna" .)