Karate achievement that you can't build on? (5,4)
I believe the answer is:
green belt
'karate achievement that you can't' is the definition.
The definition suggests a singular noun which matches the answer.
'on?' is the wordplay.
I cannot quite understand how this works, but
'on?' could be 're' (regarding) and 're' is found in the answer.
This explanation may well be incorrect...
'build' is the link.
Can you help me to learn more?
(Other definitions for green belt that I've seen before include "of restricted development?" , "Something about London" , "Martial artist" , "land embracing a community?" , "status of karate?" .)