Not left? That's correct! (5)
I believe the answer is:
right
I believe this is a double definition.
'not left?' is the first definition.
('right' is the opposite to 'left')
'correct' is the second definition.
(right can mean correct)
'that's' acts as a link.
(Other definitions for right that I've seen before include "licence" , "Equitable" , "Fit" , "Fitting; entitlement" , "one side" .)