Him? Prone to change opium alkaloid? (8)
I believe the answer is:
morphine
'opium alkaloid?' is the definition.
Although both the answer and definition are singular nouns, I don't see how one could define the other.
'him? prone to change' is the wordplay.
'to change' is an anagram indicator.
'him'+'prone'='himprone'
'himprone' anagrammed gives 'MORPHINE'.
Can you help me to learn more?
(Other definitions for morphine that I've seen before include "Pain-relieving opiate" , "Pain-easing opiate" , "Opiate for pain relief" , "Sedative derived from opium" , "Nip Homer in the bud and you'll get a powerful painkiller" .)