Yearn to be extensive (4)
I believe the answer is:
long
I believe this clue is a double definition.
'yearn' is the first definition.
(synonyms)
'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 "Really want" , "Pant" , "Yearn (for)" , "Sustained" , "Thus having been treated by stretcher, perhaps" .)