Inject old intramuscular medium with it (5)
I believe the answer is:
immit
'inject old' is the definition.
I know nothing about this answer so I cannot judge whether this works.
'intramuscular medium with it' is the wordplay.
'intramuscular medium' becomes 'imm' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'with' is a charade indicator (letters next to each other).
'imm'+'it'='IMMIT'
Can you help me to learn more?
(Another definition for immit that I've seen is " No longer introduce".)