Chances overdose before drugs case (4)
I believe the answer is:
odds
'chances' is the definition.
(thesaurus)
'overdose before drugs case' is the wordplay.
'overdose' becomes 'od' ('o.d.' can be a synonym of 'overdose').
'before' is a charade indicator (letters next to each other).
'case' says to hollow out the word (remove centre letters).
'drugs' with its middle removed is 'ds'.
'od'+'ds'='ODDS'
(Other definitions for odds that I've seen before include "Ratios set by bookies" , "Chances expressed as a ratio" , "Betting price" , "Chance" , "Perhaps even money" .)