Joke about girl being straitlaced (7)
I believe the answer is:
puritan
'straitlaced' is the definition.
(thesaurus)
'joke about girl' is the wordplay.
'joke' becomes 'pun' (pun is a type of joke).
'about' indicates putting letters inside.
'girl' becomes 'rita' (woman's name).
'pun' enclosing 'rita' is 'PURITAN'.
'being' acts as a link.
(Other definitions for puritan that I've seen before include "Person of extreme moral strictness" , "Strict person for whom pleasure is sinful" , "Over-strict moralist" , "Upright" , "Adherent ot strict religous principles, opposed to sensual pleasures" .)