Hard to undo; drunk (5)
I believe the answer is:
tight
This clue is a double definition.
'hard to undo' is the first definition.
(I've seen this before)
'drunk' is the second definition.
(slang term)
(Other definitions for tight that I've seen before include "Strictly imposed; drunk" , "Miserly (informal) - drunk (informal)" , "Difficult and problematic" , "Mean - drunk" , "Close-fitting; tense" .)