Substance turning up in drug ingested by couriers (8)
I believe the answer is:
selenium
'substance' is the definition.
(selenium is a kind of substance)
'turning up in drug ingested by couriers' is the wordplay.
'turning up' says the letters should be written backwards.
'drug' becomes 'e' (e is another name for ecstasy).
'ingested by' means one lot of letters goes inside another.
'couriers' becomes 'mules' (mule can informally mean a courier of drugs).
'in'+'e'='ine'
'ine' inserted within 'mules' is 'muineles'.
'muineles' back-to-front is 'SELENIUM'.
(Another definition for selenium that I've seen is " Semiconducting element, symbol Se".)