Part of New York marathon is alight (4,6)
I believe the answer is:
long island
'part of new york' is the definition.
('Long Island' is part of 'new york')
'marathon is alight' is the wordplay.
'marathon' becomes 'long' (marathon can mean long or lengthy).
'alight' becomes 'land' (I've seen this in another clue).
'long'+'is'+'land'='LONG ISLAND'
(Other definitions for long island that I've seen before include "part of America" , "Part of New York" , "patch cut off in New York" .)