Nothing left? That's OK (3,5)
I believe the answer is:
all right
This clue is a double definition.
'nothing left?' is the first definition.
(I've seen this before)
'ok' is the second definition.
(synonyms)
'that's' is the link.
(Other definitions for all right that I've seen before include "Of acceptable quality" , "Very well, as you wish" , "Satisfactory; allowable" , "Very well, everything's ok" , "Safe - adequate" .)