Finished badly on Ecstasy tablet (8)
I believe the answer is:
pastille
'tablet' is the definition.
(I've seen this before)
'finished badly on ecstasy' is the wordplay.
'finished' becomes 'past'.
'badly' becomes 'ill' ('ill' can be adverbial as in 'ill-advised').
'on' says to put letters next to each other.
'ecstasy' becomes 'e' (informal term for the drug).
'past'+'ill'+'e'='PASTILLE'
(Other definitions for pastille that I've seen before include "Small, often medicated, sweet" , "One medicated sweet is the alternative" , "tablet" , "Flavoured or medicated lozenge" , "drop" .)