Joke about girl, prude (7)
I believe the answer is:
puritan
'prude' is the definition.
(synonyms)
'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' placed around 'rita' is 'PURITAN'.
(Other definitions for puritan that I've seen before include "Extreme moralist" , "Person of extreme moral strictness" , "Person of extremely strict morals" , "Killjoy" , "Upright" .)