Cardin's sin cutting pie? (6)
I believe the answer is:
pierre
'cardin's' is the definition.
I can't tell whether this defines the answer.
'sin cutting pie?' is the wordplay.
'sin' becomes 'err' (to sin is to err morally).
'cutting' is an insertion indicator (some letters cut their way into a word).
'err' placed within 'pie' is 'PIERRE'.
Can you help me to learn more?
(Other definitions for pierre that I've seen before include "One colouring cloth" , "Peter, to the French" , "French boy's name" , "See 17" , "French first name" .)