Old bloke in New York, a nonentity (6)
I believe the answer is:
nobody
'a nonentity' is the definition.
('nobody' can be a synonym of 'nonentity')
'old bloke in new york' is the wordplay.
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'bloke' becomes 'bod' (I've seen this in other clues).
'in' is an insertion indicator.
'new york' becomes 'ny'.
'o'+'bod'='obod'
'obod' put within 'ny' is 'NOBODY'.
(Other definitions for nobody that I've seen before include "Literary diarist" , "problem for Morse?" , "Eg Pooter" , "Not a single person" , "Unimportant person" .)