Metacarpal remedied with injection of ordinary drug (11)
I believe the answer is:
paracetamol
'drug' is the definition.
(I know that paracetamol is a drug)
'metacarpal remedied with injection of ordinary' is the wordplay.
'remedied' is an anagram indicator.
'with injection of' is an insertion indicator (some letters injected in).
'ordinary' becomes 'O' (abbreviation eg in O-Level, Ordinary Level).
'metacarpal' with letters rearranged gives 'paracetaml'.
'paracetaml' going around 'o' is 'PARACETAMOL'.
(Other definitions for paracetamol that I've seen before include "Common analgesic" , "Common remedy" , "Number" , "Pain/fever reliever" , "Synthetic painkiller" .)