Island I leave to probe US city villainy (8)
I believe the answer is:
iniquity
'villainy' is the definition.
(thesaurus)
'island i leave to probe us city' is the wordplay.
'island' becomes 'i' (geographical abbreviation).
'leave' becomes 'quit' (quitting is a kind of leaving).
'to probe' is an insertion indicator (probe can mean to poke into).
'us city' becomes 'NY' (New York).
'i'+'quit'='iquit'
'iquit' going within 'ny' is 'niquity'.
'i'+'niquity'='INIQUITY'
(Other definitions for iniquity that I've seen before include "Evil act" , "Going wrong" , "Evil, wickedness" , "Heinousness" , "Great wickedness or injustice" .)