Sounds like size zero is too much (6)
I believe the answer is:
excess
'too much' is the definition.
(excess of something means too much of it)
I don't understand the rest of the clue.
Can you help me to learn more?
(Other definitions for excess that I've seen before include "Too much - going too far" , "A quantity larger than is needed or immoderation" , "Immoderation as in eating, too much" , "Amount left over" , "Intemperance" .)