The spirit that let priest go free? (11)
I believe the answer is:
poltergeist
'the spirit' is the definition.
(poltergeist is a kind of spirit)
'let priest go free?' is the wordplay.
'free?' indicates an anagram (I've seen 'free' mean this).
'let'+'priest'+'go'='letpriestgo'
'letpriestgo' is an anagram of 'POLTERGEIST'.
'that' acts as a link.
(Other definitions for poltergeist that I've seen before include "disruptive presence" , "Noisy spook" , "Active ghost" , "Mischievous ghost" , "Noisy ghost" .)