Drinker really after drop (5)
I believe the answer is:
dipso
'drinker' is the definition.
(I have seen 'Habitual drinker' mean 'dipso' so perhaps 'drinker' could also mean 'dipso')
'really after drop' is the wordplay.
'really' becomes 'so' (both can mean 'extremely').
'after' is a charade indicator (letters next to each other).
'drop' becomes 'dip' (I've seen this before).
'so' after 'dip' is 'DIPSO'.
(Other definitions for dipso that I've seen before include "lush" , "drunk" , "(An) alcoholic (colloq.)" , "Habitual drinker" , "A habitual 18" .)