Reprieve is equal to professor (6)
I believe the answer is:
pardon
'reprieve' is the definition.
(thesaurus)
'equal to professor' is the wordplay.
'equal' becomes 'par'.
'to professor' becomes 'don' (I've seen this before).
'par'+'don'='PARDON'
'is' is the link.
(Other definitions for pardon that I've seen before include "Excuse or absolve" , "Allow to go unpunished" , "Polite expression of regret, please excuse" , "What was that you said" , "Forgive (an error)" .)