One is so unusual (3)
I believe the answer is:
odd
I believe this is a double definition.
'one is' is the first definition.
(I've seen this before)
'unusual' is the second definition.
(similar in meaning)
'so' acts as a link.
(Other definitions for odd that I've seen before include "It's certainly not even" , "Peculiar or eccentric" , "Left over" , "like half the numbers?" , "Extraordinary" .)