First person's having tart with treacly coating - what sinfulness! (7)
I believe the answer is:
impiety
'sinfulness' is the definition.
(thesaurus)
'first person's having tart with treacly coating' is the wordplay.
I cannot quite see how this works, but
'first' could be 'i' (I in Roman numerals eg Elizabeth I) and 'i' is located in the answer.
'tart' could be 'pie' (both are baked goods) and 'pie' is found in the answer.
This may be the basis of the clue (or it may be nonsense).
'what' is the link.
Can you help me to learn more?
(Other definitions for impiety that I've seen before include "Irreligion" , "Irreverence, profanity" , "godlessness" , "Lack of reverence" , "sacrilege" .)