Day in NY? Yes, fantastic city (6)
I believe the answer is:
sydney
'city' is the definition.
(Sydney is an example)
'day in ny? yes fantastic' is the wordplay.
'day' becomes 'd' (abbreviation for day).
'in' means one lot of letters goes inside another.
'fantastic' is an anagram indicator (a fantastic or improbable ordering of the letters).
'ny'+'yes'='nyyes'
'nyyes' is an anagram of 'syney'.
'd' inserted inside 'syney' is 'SYDNEY'.
(Other definitions for sydney that I've seen before include "- - Carton, 9 hero" , "State capital of New South Wales" , "First name" , "City with famous harbour bridge" , "Largest Australian city" .)