Drug of poppy in use medicinally, initially (5)
I believe the answer is:
opium
'drug' is the definition.
(opium is a kind of drug)
'of poppy in use medicinally initially' is the wordplay.
'of' becomes 'o'.
'initially' says to take the initial letters.
The first letters of 'poppy in use medicinally' is 'pium'.
'o'+'pium'='OPIUM'
(Other definitions for opium that I've seen before include "Poppy-based drug" , "Narcotic drug" , "Duller" , "Poppy drug" , "Narcotic derived from poppy" .)