Get high on heroin. Injecting one mil makes body spasm (6)
I believe the answer is:
hiccup
'spasm' is the definition.
(I have seen 'Involuntary spasm ' mean 'hiccup' so perhaps 'spasm' could also mean 'hiccup')
'get high on heroin injecting one mil makes body' is the wordplay.
I cannot quite see how this works, but
'high' could be 'up' (associated in meaning) and 'up' is present in the answer.
'heroin' could be 'h' (H is a slang term for heroin) and 'h' is located in the answer.
'one' could be 'i' (Roman numeral) and 'i' is found within the answer.
'mil' could be 'cc' (synonyms) and 'cc' is located in the answer.
This accounts for all the letters.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for hiccup that I've seen before include "Spasm of the diaphragm or an informal hitch" , "Minor problem" , "slight setback" , "Minor hitch" , "Reflex spasm of the diaphragm" .)