It's unusual and there's only one (8)
I believe the answer is:
singular
'it's unusual' is the definition.
(I've seen this before)
'there's only one' is the wordplay.
I cannot quite see how this works, but
'there's' could be 'n' (I've seen this in other clues) and 'n' is found in the answer.
'one' could be 'i' (Roman numeral) and 'i' is found in the answer.
This explanation may well be incorrect...
'and' is the link.
Can you help me to learn more?
(Other definitions for singular that I've seen before include "Referring to one thing" , "Unusual, striking, or unique" , "Remarkable; not plural" , "Denoting one" , "distinctive" .)