A medic injecting heroin as needed (2,3)
I believe the answer is:
ad hoc
'needed' is the definition.
Both the definition and answer are adjectives. Perhaps they are linked in a way I don't understand?
'a medic injecting heroin' is the wordplay.
'medic' becomes 'doc' (doctor).
'injecting' means one lot of letters goes inside another.
'heroin' becomes 'H' (H is a slang term for heroin).
'doc' placed around 'h' is 'dhoc'.
'a'+'dhoc'='AD HOC'
'as' is the link.
Can you help me to learn more?
(Other definitions for ad hoc that I've seen before include "For one specific case" , "For this special purpose (Latin)" , "Intended for one specific matter" , "not in general practice" , "particular case" .)