The odd unknown (7)
I believe the answer is:
strange
This clue is a double definition.
'the odd' is the first definition.
(thesaurus)
'unknown' is the second definition.
('strange' is associated in meaning with 'unknown')
(Other definitions for strange that I've seen before include "Peculiar; Morse's boss" , "Extraordinary" , "Out of the ordinary" , "Queer" , "Foreign" .)