A New Yorker's great too (2,4)
I believe the answer is:
as well
'too' is the definition.
(synonyms)
'a new yorker's great' is the wordplay.
I cannot quite see how this works, but
'a' is within the answer.
'great' could be 'swell' (synonyms) and 'swell' is present in the answer.
No letters remain.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for as well that I've seen before include "A swell sort of addition" , "on top" , "with equal effect" , "Also, in addition" , "To boot" .)