Firmly maintain it’s wrong to admit mortal sin (6)
I believe the answer is:
insist
'firmly maintain' is the definition.
'insist' can be an answer for 'maintain' (maintain can mean to insist something is true). I am unsure of the 'firmly' bit.
'it's wrong to admit mortal sin' is the wordplay.
'wrong' indicates anagramming the letters.
'to admit' indicates putting letters inside.
'mortal sin' becomes 'nsi' (I can't explain this - if you can you should give a lot more credence to this answer).
'its' is an anagram of 'ist'.
'ist' enclosing 'nsi' is 'INSIST'.
Can you help me to learn more?
(Other definitions for insist that I've seen before include "Declare or demand emphatically" , "Assert firmly" , "Continue to assert, maintain" , "Be firm in demands" , "Put one's foot down" .)