The basis of judo, etc. (3)
I believe the answer is:
mat
'the basis' is the definition.
Although both the answer and definition are singular nouns, I cannot see how one could define the other.
I cannot understand how the rest of the clue works.
Can you help me to learn more?
(Other definitions for mat that I've seen before include "Opening of good book" , "coaster" , "Small protective floor-covering" , "Tangle" , "Floor-pad" .)