Numbers swallowing a lithium drug (9)
I believe the answer is:
digitalis
'drug' is the definition.
(I've seen this before)
'numbers swallowing a lithium' is the wordplay.
'numbers' becomes 'digits' (digit is a kind of number**).
'swallowing' indicates putting letters inside.
'lithium' becomes 'li' (Li is the chemical symbol for lithium).
'a'+'li'='ali'
'digits' enclosing 'ali' is 'DIGITALIS'.
(Other definitions for digitalis that I've seen before include "Drug from foxgloves" , "Heart drug prepared from foxglove leaves" , "Cardiac stimulant prepared from 11" , "plant making this drug?" , "medicinal drug" .)