Exceedingly uncommon (4)
I believe the answer is:
rare
'exceedingly uncommon' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for rare that I've seen before include "Choice - lightly cooked - thin" , "Might a four-leaf clover be so" , "Not common" , "Infrequent; remarkable" , "like Ben Jonson, Jack Young thought" .)