Rob chap on island thoroughfare (5)
I believe the answer is:
heist
'rob' is the definition.
(I know that rob is a more specific form of the action heist)
'chap on island thoroughfare' is the wordplay.
'chap' becomes 'he'.
'on' means one lot of letters go next to another (in a down clue, letters appear on others).
'island' becomes 'i' (geographical abbreviation).
'thoroughfare' becomes 'st' (I've seen this in other clues).
'he'+'i'+'st'='HEIST'
(Other definitions for heist that I've seen before include "Mugging" , "Armed robbery" , "Armed hold-up (sl.)" , "bank job in America?" , "crime?" .)