Yearn to be extensive (4)
I believe the answer is:
Here is my best explanation:
This clue is a double definition.
'yearn' is the first definition.
('long' can be a synonym of 'yearn')
'extensive' is the second definition.
(I've seen this before)
'to be' is the link.
(Other definitions for long that I've seen before include "Extended", "Protracted", "Yearn (for)", "Extensive - pine" and "Lengthy - pine".)