Not drunk? That's restrained! (5)
I believe the answer is:
sober
I believe this is a double definition.
'not drunk?' is the first definition.
(I've seen this before)
'restrained' is the second definition.
(I've seen this before)
'that's' acts as a link.
(Other definitions for sober that I've seen before include "Dignified and serious" , "Serious, dull" , "Earnest" , "Like a judge?" , "Staid; clear-headed" .)