Mitigation shown by judge locking up priest (6)
I believe the answer is:
relief
'mitigation' is the definition.
(I've seen this before)
'judge locking up priest' is the wordplay.
'judge' becomes 'ref' (referee).
'locking up' is an insertion indicator (I've seen this in other clues).
'priest' becomes 'eli' (biblical figure).
'ref' placed around 'eli' is 'RELIEF'.
'shown by' acts as a link.
(Other definitions for relief that I've seen before include "Freeing of besieged city" , "Removal of stress" , "Alleviation or help" , "Stand-in" , "Lessening or removal of pain" .)