After injecting drug, parent is numb (4)
I believe the answer is:
dead
'numb' is the definition.
(I've seen this before)
'after injecting drug' is the wordplay.
'after' becomes 'dad' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'injecting' indicates putting letters inside.
'drug' becomes 'e' (e is another name for ecstasy).
'dad' going around 'e' is 'DEAD'.
'parent is' is the link.
I am not very happy about this link. Some or all of it may belong to another bit of the clue.
Can you help me to learn more?
(Other definitions for dead that I've seen before include "Defunct" , "Stiff - bereft of life" , "Absolutely" , "Exhausted -- layabout" , "Deceased; numb" .)