Episcopal headgear finally cast in mud (5)
I believe the answer is:
'episcopal headgear' is the definition.
(headgear worn by bishops)
'finally cast in mud' is the wordplay.
'finally' indicates one should take the final letters.
'in' means one lot of letters goes inside another.
'mud' becomes 'mire' (synonyms).
The last letter of 'cast' is 't'.
't' put within 'mire' is 'MITRE'.
(Other definitions for mitre that I've seen before include "Bishop's tall headdress" , "Clerical coverage" , "Bishop's headwear" , "Corner joint - bishop's headdress" , "Remit (anag.)" .)