Finish with hip or fashionable new painkiller (9)
I believe the answer is:
endorphin
'painkiller' is the definition.
(I know that endorphin is a opioid)
'finish with hip or fashionable new' is the wordplay.
'finish' becomes 'end' (synonyms).
'with' is a charade indicator (letters next to each other).
'fashionable' indicates an anagram (letters can be fashioned into something else).
'new' becomes 'n' (common abbreviation eg NT for New Testament).
'hip'+'or'='hipor'
'hipor' anagrammed gives 'orphi'.
'end'+'orphi'+'n'='ENDORPHIN'
(Other definitions for endorphin that I've seen before include "Opiate-like substance produced in the brain" , "Neurochemical with analgesic properties - dinner hop (anag)" , "painkiller" , "Hormone in the brain with an analgesic effect" .)