Joke about girl being strictly moral (7)
I believe the answer is:
puritan
'strictly moral' is the definition.
'joke about girl' is the wordplay.
'joke' becomes 'pun' (pun is a type of joke).
'about' means one lot of letters goes inside another.
'girl' becomes 'rita' (woman's name).
'pun' going around 'rita' is 'PURITAN'.
'being' acts as a link.
(Other definitions for puritan that I've seen before include "Upright" , "Prude" , "Person of censorious moral beliefs" , "Killjoy" , "Member of a group of English Protestants who thought that the Protestant Reformation was incomplete" .)