As much of the cancan as one is able to do (4)
I believe the answer is:
half
'as much' is the definition.
The answer and definition can be both quantities as well as being singular nouns.
Perhaps you can see a link between them that I can't see?
I can't explain the rest of the clue.
Can you help me to learn more?
(Other definitions for half that I've seen before include "one over a couple" , "One of two equal parts" , "shared round" , "Short distance - steal!" , "alcoholic drink" .)