Yearn to be extensive (4)
I believe the answer is:
long
I believe this is a double definition.
'yearn' is the first definition.
(I know that yearn can be written as long)
'be extensive' is the second definition.
(I've seen this before)
'to' acts as a link.
(Other definitions for long that I've seen before include "Really want" , "Pant" , "Yearn (for)" , "Sustained" , "Thus having been treated by stretcher, perhaps" .)