Foolishly, a cop plays with drug, leading to disaster (10)
I believe the answer is:
apocalypse
'to disaster' is the definition.
(apocalypse is a kind of disaster)
'foolishly a cop plays with drug leading' is the wordplay.
'foolishly' indicates anagramming the letters.
'with' is a charade indicator (letters next to each other).
'drug' becomes 'e' (E is another name for ecstasy).
'leading' means one lot of letters go next to another.
'a'+'cop'+'plays'='acopplays'
'acopplays' anagrammed gives 'apocalyps'.
'apocalyps'+'e'='APOCALYPSE'
(Other definitions for apocalypse that I've seen before include "terrible event" , "Violent event" , "Revelation" , "World's end" , "Catastrophic event" .)