Drunk after drink is unassailable (10)
I believe the answer is:
watertight
This could be a double definition.
'drunk after drink' is the first definition.
The answer and definition are different parts of speech. However, past participle verbs and adjectives can occasionally define each other.
'unassailable' is the second definition.
('watertight' can be a synonym of 'unassailable')
'is' is the link.
Can you help me to learn more?
(Other definitions for watertight that I've seen before include "Proof against leaking" , "Without loopholes" , "Unassailable" , "Closely sealed" , "Impregnable" .)