Nothing left? That’s OK (3,5)
I believe the answer is:
all right
I believe 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' acts as a 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" .)