A synthetic leper mixes with pony (9)

I believe the answer is:

propylene

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

'leper mixes with pony' is the wordplay.
'mixes with' indicates an anagram.
'leper' with its letters mixed up with 'pony' can make 'PROPYLENE'.

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

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