One is curious (8)
I believe the answer is:
singular
I believe this is a double definition.
'one' is the first definition.
('singular' can be a synonym of 'one')
'curious' is the second definition.
('singular' can be a synonym of 'curious')
'is' is the link.
(Other definitions for singular that I've seen before include "Strange - unique" , "Unusual - not plural" , "number" , "One of a kind" , "Extraordinary - conspicuous - sole" .)