A drink good for arresting one’s final degeneration (6)
I believe the answer is:
ageing
'degeneration' is the definition.
(degenerating as you age)
'a drink good for arresting one's final' is the wordplay.
'drink' becomes 'gin' (gin is a kind of drink).
'good' becomes 'g' (abbreviation).
'for arresting' indicates putting letters inside.
'final' indicates one should take the final letters.
The last letter of 'one' is 'e'.
'gin'+'g'='ging'
'ging' going around 'e' is 'geing'.
'a'+'geing'='AGEING'
(Other definitions for ageing that I've seen before include "Growing old" , "Declining" , "Elderly" , "Growing older" , "rather old" .)