It's religiously worn in a certain joint (5)
I believe the answer is:
mitre
'a certain joint' is the definition.
'mitre' can be an answer for 'joint' (mitre joint is a kind of joint). I am not sure about the 'a certain' bit.
'it's religiously worn in' is the wordplay.
'it' becomes ''t' (abbreviation. e.g. in 'tis).
'religiously worn' becomes 'mire' (I can't justify this - if you can you should give a lot more credence to this answer).
'in' means one lot of letters goes inside another.
't' going within 'mire' is 'MITRE'.
Can you help me to learn more?
(Other definitions for mitre that I've seen before include "top?" , "bishop's head?" , "A bishop wears it" , "Sort of joint" , "What a bishop may have on" .)