Using numbers is a drug (9)
I believe the answer is:
digitalis
'a drug' is the definition.
(I know that digitalis is a type of drug)
'using numbers is' is the wordplay.
'using numbers' becomes 'digital' (I can't justify this - if you can you should give a lot more credence to this answer).
'digital'+'is'='DIGITALIS'
Can you help me to learn more?
(Other definitions for digitalis that I've seen before include "plant making this drug?" , "Heart drug prepared from foxglove leaves" , "medicinal drug" , "Drug from foxgloves" , "Drug derived from foxglove leaves" .)