Probability of overdoses ingesting drug's source (4)
I believe the answer is:
odds
'probability' is the definition.
(I know that odds is a type of likelihood)
'overdoses ingesting drug's source' is the wordplay.
'overdoses' becomes 'o.d.s' ('o.d.' can be a synonym of 'overdose').
'ingesting' is an insertion indicator.
'source' suggests taking the first letters (source can mean a beginning or origin).
The first letter of 'drug' is 'd'.
'ods' going around 'd' is 'ODDS'.
'of' acts as a link.
(Other definitions for odds that I've seen before include "(In betting) chances" , "Partial darkness" , "Indicator of likely betting winnings" , "The chances of something happening" , "Perhaps even money" .)