Lightly cooked? It’s not common (4)
I believe the answer is:
rare
I believe this is a double definition.
'lightly cooked?' is the first definition.
(as in a rare steak)
'not common' is the second definition.
('rare' is the opposite to 'common')
'it's' acts as a link.
(Other definitions for rare that I've seen before include "Still red inside (as hens' teeth?)" , "Meat that's lightly cooked" , "Very scarce, like undercooked steak" , "Excellent; uncommon" , "Unusual(ly fine)" .)