Habitual drinker (5)
I believe the answer is:
toper
'habitual drinker' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for toper that I've seen before include "One who drinks to excess" , "Serious drinker" , "Trope for habitual drinker" , "Heavy drinker" , "Someone with a drink problem" .)