Drug injected primarily in a poet being dissolute (6)
I believe the answer is:
opiate
'drug' is the definition.
(opiate is a kind of drug)
'injected primarily in a poet being dissolute' is the wordplay.
'primarily' indicates taking the first letters.
'in' indicates putting letters inside.
'being dissolute' indicates anagramming the letters.
The first letter of 'injected' is 'i'.
'a'+'poet'='apoet'
'apoet' is an anagram of 'opate'.
'i' inserted inside 'opate' is 'OPIATE'.
(Other definitions for opiate that I've seen before include "of heroin, perhaps" , "Preparation made from poppy derivative" , "A poet I become on this drug" , "Sedating drug" , "Narcotic, such as morphine" .)