Old judge will imprison one, that’s clear (9)
I believe the answer is:
exonerate
'clear' is the definition.
('exonerate' can be a synonym of 'clear')
'old judge will imprison one' is the wordplay.
'old' becomes 'ex'.
'judge' becomes 'rate' (rating is a kind of judging).
'will imprison' indicates putting letters inside.
'ex'+'rate'='exrate'
'exrate' going around 'one' is 'EXONERATE'.
'that's' acts as a link.
(Other definitions for exonerate that I've seen before include "Clear of blame" , "let off the hook" , "Pronounce not guilty of a crime" , "Discharge" , "Free of duty" .)