Overdose on drink, principally rum (3)
I believe the answer is:
odd
'rum' is the definition.
(thesaurus)
'overdose on drink principally' is the wordplay.
'overdose' becomes 'od' ('o.d.' can be a synonym of 'overdose').
'on' means one lot of letters go next to another.
'principally' indicates taking the first letters.
The first letter of 'drink' is 'd'.
'od'+'d'='ODD'
(Other definitions for odd that I've seen before include "One is" , "Strange, bizarre" , "Freakish" , "Weird" , "Unmatched" .)