In my sin, maybe, there's nothing but bad practice (6)
I believe the answer is:
simony
'bad practice' is the definition.
The answer and definition can be both acts as well as being singular nouns.
Perhaps you can see an association between them that I don't see?
'in my sin maybe there's nothing' is the wordplay.
'in' means one lot of letters goes inside another.
'maybe' is an anagram indicator.
'there's nothing' becomes 'o' (I've seen this in other clues).
'my'+'sin'='mysin'
'mysin' anagrammed gives 'simny'.
'simny' going around 'o' is 'SIMONY'.
'but' acts as a link.
Can you help me to learn more?
(Other definitions for simony that I've seen before include "The buying or selling of ecclesiastical privileges" , "cloth trade" , "Dealing in church property" , "purchase of church office" , "Trade in ecclesiastical pardons" .)