What's more odd, you've never seen him before (8)
I believe the answer is:
stranger
'you've never seen him before' is the definition.
The answer is a person as well as being a singular noun. This is suggested by the definition.
'what's more odd' is the wordplay.
I cannot really see how this works, but
'odd' could be 'strange' (I've seen this before) and 'strange' is found in the answer.
A single letter 'r' remains which might be clued in a way I don't see.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for stranger that I've seen before include "More odd or weird" , "Alien, foreigner" , "The unknown one is more unusual" , "Non-acquaintance" , "Outsider" .)