### Clue: Gas results from cook quitting heroin substitute (7)
### Answer: METHANE 
 ### Explanation: Definition: Gas
 "methadone"(a substitute drug for heroin, used in the treatment of heroin addiction) **minus**(… quitting) "do"(to cook food to a degree required by the consumer, as in "How'd you like me to do your steak – rare, medium or well done or anywhere in between?").