German article describing particular tranquilliser (6)
I believe the answer is:
downer
'tranquilliser' is the definition.
(I've seen this before)
'german article describing particular' is the wordplay.
'german article' becomes 'der' (definite article in German).
'describing' is an insertion indicator (describe can mean to form the outline of).
'particular' becomes 'own' (I've seen this in other clues).
'der' going around 'own' is 'DOWNER'.
(Other definitions for downer that I've seen before include "Depressing experience" , "Sedative" , "Saddening experience" , "Informally a depressant drug" , "tranquilliser" .)