Drunk? That’s illegal (5,3,5)
I believe the answer is:
under the table
I believe this clue is a double definition.
'drunk?' is the first definition.
(thesaurus)
'illegal' is the second definition.
(thesaurus)
'that's' acts as a link.
(Other definitions for under the table that I've seen before include "totally gone" , "Dodgy" , "Too tight" , "secretly" , "at the bottom of the list" .)