Fraud unknown on south-west island (4)
I believe the answer is:
'fraud' is the definition.
(I know that swiz is a type of cheat)
'unknown on south-west island' is the wordplay.
'unknown' becomes 'z' (algebra).
'on' says to put letters next to each other.
'south west' becomes 'sw'.
'island' becomes 'i' (geographical abbreviation).
'z' put after 'swi' is 'SWIZ'.
(Other definitions for swiz that I've seen before include "con" , "Sharp practice" , "Fraud (slang)" .)