Trace rascal smuggling ecstasy and heroin (6)
I believe the answer is:
breath
'trace' is the definition.
(thesaurus)
'rascal smuggling ecstasy and heroin' is the wordplay.
'rascal' becomes 'brat' (brat is a kind of rascal).
'smuggling' indicates putting letters inside (I've seen this in other clues).
'ecstasy' becomes 'e' (informal term for the drug).
'and' means one lot of letters go next to another.
'heroin' becomes 'h' (synonyms).
'brat' enclosing 'e' is 'breat'.
'breat'+'h'='BREATH'
(Other definitions for breath that I've seen before include "Air received and expelled" , "Bertha (anag.)" , "from which inspiration may be drawn" , "Bather (anag.)" , "An inhalation and exhalation" .)