New York location that's busy by nine, perhaps (5,6)
I believe the answer is:
times square
'new york location that's busy' is the definition.
'by nine perhaps' is the wordplay.
'by' becomes 'times' (I've seen this before).
'nine perhaps' becomes 'square' (nine is a square number).
'times'+'square'='TIMES SQUARE'
(Other definitions for times square that I've seen before include "New York landmark" , "Perhaps this grid" , "Manhattan area (18?)" , "found in New York" , "place in New York?" .)