Unassailable drunk needs drink first (10)
I believe the answer is:
watertight
'unassailable' is the definition.
(I know that unassailable can be written as watertight)
'drunk needs drink first' is the wordplay.
'drunk' becomes 'tight' (slang term).
'needs drink' becomes 'water' (water is an example. I am not sure about the 'needs' bit.).
'first' is a charade indicator (letters next to each other).
'tight' put after 'water' is 'WATERTIGHT'.
Can you help me to learn more?
(Other definitions for watertight that I've seen before include "Proof against leaking" , "Closely sealed" , "seaworthy" , "Without loopholes" , "Impermeable - unbreakable" .)