Old magistrate imprisoning Prince I let off (8)
I believe the answer is:
reprieve
'let off' is the definition.
(thesaurus)
'old magistrate imprisoning prince i' is the wordplay.
'old magistrate' becomes 'reeve' (I've seen this before).
'imprisoning' is an insertion indicator.
'prince' becomes 'pr'.
'pr'+'i'='pri'
'reeve' placed around 'pri' is 'REPRIEVE'.
(Other definitions for reprieve that I've seen before include "Pardon - respite" , "Delay or cancel (death sentence)" , "Postponement of trouble or sentence" , "Allow to go unpunished" , "Temporary relief, like stay on death sentence" .)