Proverbial drinker with yen for rum (5)
I believe the answer is:
fishy
'for rum' is the definition.
(both can mean suspicious or odd)
'proverbial drinker with yen' is the wordplay.
'proverbial drinker' becomes 'fish' (as in the phrase 'drink like a fish').
'with' is a charade indicator (letters next to each other).
'yen' becomes 'y' (resembles the symbol for the currency of Japan).
'fish'+'y'='FISHY'
(Other definitions for fishy that I've seen before include "As smelt, perhaps" , "Questionable" , "Arousing suspicion" , "Ray is so" , "Dodgy, suspicious" .)