Quick one on foot in New York (5)
I believe the answer is:
nifty
'quick' is the definition.
(thesaurus)
'one on foot in new york' is the wordplay.
'one' becomes 'i' (Roman numeral).
'on' says to put letters next to each other (in a down clue, letters appear on others).
'foot' becomes 'ft' (abbreviation).
'in' means one lot of letters goes inside another.
'new york' becomes 'ny'.
'i'+'ft'='ift'
'ift' placed inside 'ny' is 'NIFTY'.
(Other definitions for nifty that I've seen before include "Clever; stylish" , "Smart" , "Slick" , "Quick - stylish" , "Stylish, nimble" .)