Endanger old New York accord (7)
I believe the answer is:
harmony
'accord' is the definition.
(I've seen this before)
'endanger old new york' is the wordplay.
'endanger' becomes 'harm' (harm can mean to put in danger).
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'new york' becomes 'ny'.
'harm'+'o'+'ny'='HARMONY'
(Other definitions for harmony that I've seen before include "Mutual rapport" , "Pleasing blend of sounds" , "Agreement, concord" , "sounds agreeable" , "Mellifluous sound" .)