The occasional one without a mate (3)
I believe the answer is:
odd
I believe this clue is a triple definition!
'the occasional' is the first definition.
(odd can mean occasional or infrequent)
'one' is the second definition.
(I've seen this before)
'without a mate' is the third definition.
(I've seen this in another clue)
(Other definitions for odd that I've seen before include "Extraordinary" , "Not matching" , "Freakish" , "Eccentric or unpaired" , "Weird" .)