Greatest numbers going without drug (9)
I believe the answer is:
digitalis
'drug' is the definition.
(I know that digitalis is a type of drug)
'greatest numbers going without' is the wordplay.
I cannot really understand how this works, but
'numbers' could be 'is' (I is an example) and 'is' is located in the answer.
The remaining letters 'digital' is a valid word which might be clued in a way I don't see.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for digitalis that I've seen before include "Drug from foxgloves" , "Heart drug prepared from foxglove leaves" , "plant making this drug?" , "Cardiac stimulant prepared from 11" , "medicinal drug" .)