Fraud unknown on south-west island (4)
I believe the answer is:
swiz
'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).
'sw'+'i'='swi'
'z' put after 'swi' is 'SWIZ'.
(Other definitions for swiz that I've seen before include "con" , "Sharp practice" , "Fraud (slang)" .)