Joint that's ending covered in mud (5)
I believe the answer is:
mitre
'joint' is the definition.
(I know that mitre is a type of joint)
'that's ending covered in mud' is the wordplay.
'ending' indicates one should take the final letters (the ending of the word).
'covered in' means one lot of letters goes inside another.
'mud' becomes 'mire' (synonyms).
The final letter of 'that' is 't'.
't' placed within 'mire' is 'MITRE'.
(Other definitions for mitre that I've seen before include "What a bishop may have on" , "A bishop's tall hat" , "Episcopal headdress" , "Bishop's headwear" , "One identifies bishop" .)