Catholic girl's lesson in style (5)
I believe the answer is:
class
'lesson in style' is the definition.
'class' can be an answer for 'lesson' (thesaurus). I am not sure about the 'in style' bit.
'catholic girl's' is the wordplay.
'catholic' becomes 'c' (abbreviation as in RC for Roman Catholic).
'girl' becomes 'lass' (lass is a kind of girl**).
'c'+'lass'='CLASS'
(Other definitions for class that I've seen before include "Teaching group" , "School form" , "Opportunity to learn" , "showing stylish excellence" , "Group - style" .)