Straitlaced girl enters into the joke (7)
I believe the answer is:
puritan
'straitlaced' is the definition.
(thesaurus)
'girl enters into the joke' is the wordplay.
'girl' becomes 'rita' (woman's name).
'enters into' indicates putting letters inside.
'the joke' becomes 'pun' (pun is a type of joke).
'rita' going into 'pun' is 'PURITAN'.
(Other definitions for puritan that I've seen before include "narrow-minded" , "One for whom Shaw wrote" , "Adherent ot strict religous principles, opposed to sensual pleasures" , "One with very strict moral principles" , "Person of censorious moral beliefs" .)