Drink drunk by myself, drunk? That's flat! (7)
I believe the answer is:
plateau
'flat' is the definition.
(thesaurus)
'drink drunk by myself drunk?' is the wordplay.
I cannot really see how this works, but
'drink' could be 'tea' (tea is a kind of drink) and 'tea' is found within the answer.
This may be the basis of clue (or it may be nonsense).
'that's' is the link.
(Other definitions for plateau that I've seen before include "hat" , "State of little change after a period of activity" , "High tableland" , "Table" , "Flat highland" .)