Back-to-front terrain is extensive in New York (4,6)
I believe the answer is:
long island
'in new york' is the definition.
('Long Island' is part of 'new york')
'back-to-front terrain is extensive' is the wordplay.
I cannot quite understand how this works, but
'terrain' could be 'land' (I've seen this before) and 'land' is located in the answer.
'is' is found in the answer.
'extensive' could be 'long' (I've seen this in another clue) and 'long' is located in the answer.
This accounts for all the letters.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for long island that I've seen before include "patch cut off in New York" , "part of America" , "Part of New York" .)