Nothing like this answer, and that’s flat! (10)
I believe the answer is:
horizontal
'flat' is the definition.
(similar in meaning)
'nothing like this answer and' is the wordplay.
I cannot really see how this works, but
'nothing' could be 'z' and 'z' is located in the answer.
'answer' could be 'a' (abbreviation eg in Q and A) and 'a' is found within the answer.
'and' could be 'n' (common abbreviation for 'and') and 'n' is found within the answer.
This explanation may well be incorrect...
'that's' acts as a link.
Can you help me to learn more?
(Other definitions for horizontal that I've seen before include "What this answer isn't!" , "Lying flat" , "even" , "Prone" .)