"Happy" chemical gives complete alternative fix if ingesting heroin (9)
I believe the answer is:
endorphin
'happy chemical' is the definition.
The answer and definition can be both substances as well as being singular nouns.
Perhaps you can see an association between them that I can't see?
'complete alternative fix if ingesting heroin' is the wordplay.
'complete' becomes 'end' (complete means to finish or end).
'alternative' becomes 'or'.
'fix' becomes 'pin' (to pin is to fix in place).
'if ingesting' indicates putting letters inside.
'heroin' becomes 'H' (H is a slang term for heroin).
'pin' going around 'h' is 'phin'.
'end'+'or'+'phin'='ENDORPHIN'
'gives' acts as a link.
Can you help me to learn more?
(Other definitions for endorphin that I've seen before include "Opiate-like substance produced in the brain" , "Hormone in the brain with an analgesic effect" , "painkiller" , "Neurochemical with analgesic properties - dinner hop (anag)" .)