What could be drunk twice? That's irrefutable (10)
I believe the answer is:
watertight
'twice? that's irrefutable' is the definition.
I can't judge whether this defines the answer.
'what could be drunk' is the wordplay.
'what could be' becomes 'water' (I can't justify this - if you can you should believe this answer much more).
'drunk' becomes 'tight' (I've seen this in other clues).
'water'+'tight'='WATERTIGHT'
Can you help me to learn more?
(Other definitions for watertight that I've seen before include "Impregnable" , "seaworthy" , "Unassailable" , "Proof against leaking" , "Without loopholes" .)