The one responsible for mess by church? (5)
I believe the answer is:
pooch
'the one' is the definition.
Although both the answer and definition are singular nouns, I cannot understand how one could define the other.
'mess by church?' is the wordplay.
'mess' becomes 'poo' (I can't explain this - if you can you should give a lot more credence to this answer).
'by' means one lot of letters go next to another.
'church?' becomes 'ch' (common abbreviation).
'poo'+'ch'='POOCH'
'responsible for' acts as a link.
(Other definitions for pooch that I've seen before include "Doggy" , "Dog (colloq.)" , "one of our best friends" .)