What is harder to explain is I don't know him (8)
I believe the answer is:
stranger
'what is harder to explain is i don't know him' is the definition.
The answer is a person as well as being a singular noun. This is suggested by the definition.
'him' is the wordplay.
I cannot really see how this works, but
'him' could be 'ange' and 'ange' is found within the answer.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for stranger that I've seen before include "Unfamiliar person" , "Alien, foreigner" , "Newcomer, alien" , "'Fact is ........ than fiction'" , "Someone unfamiliar" .)