Important tree in New York (3,5)
I believe the answer is:
big apple
'new york' is the definition.
(I've seen this before)
'important tree' is the wordplay.
'important' becomes 'big' (synonyms).
'tree' becomes 'apple' (apple tree is a kind of tree).
'big'+'apple'='BIG APPLE'
'in' is the link.
(Other definitions for big apple that I've seen before include "Large fruit in New York?" , "New York's fruity nickname" , "NY City nickname" , "New York City (informal)" , "Slang for New York citry" .)