What may catch trespassers, some returning to island (7)
I believe the answer is:
mantrap
'what may catch trespassers' is the definition.
(a mantrap catches people)
'some returning to island' is the wordplay.
'some' becomes 'part' (only part of something).
'returning' shows that the letters should be reversed in order.
'to' says to put letters next to each other (I've seen this in other clues).
'island' becomes 'man' (the Isle of Man is an example).
'part' back-to-front is 'trap'.
'trap' after 'man' is 'MANTRAP'.
(Other definitions for mantrap that I've seen before include "poacher may have been held" , "something that ensnares folk" , "Illegal snare" , "to catch trespassers" , "gin" .)