Expert dealt out Jekyll's double chemical (12)

I believe the answer is:

acetaldehyde

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'double chemical' is the definition.
The definition and answer can be both substances as well as being singular nouns.
Perhaps there's a link between them I don't understand?

'expert dealt out jekyll's' is the wordplay.
'expert' becomes 'ace' (ace can informally mean very good).
'out' indicates anagramming the letters (out can mean wrong or inaccurate).
'jekyll' becomes 'hyde' (I can't explain this - if you can you should give a lot more credence to this answer).
'dealt' with letters rearranged gives 'talde'.
'ace'+'talde'+'hyde'='ACETALDEHYDE'

Can you help me to learn more?

I've seen this clue in the Sydney Morning Herald.

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