Joke about woman being a moralist (7)
I believe the answer is:
puritan
'a moralist' is the definition.
(I've seen this before)
'joke about woman' is the wordplay.
'joke' becomes 'pun' (pun is a type of joke).
'about' means one lot of letters goes inside another.
'woman' becomes 'rita' ().
'pun' placed around 'rita' is 'PURITAN'.
'being' acts as a link.
(Other definitions for puritan that I've seen before include "One with very strict moral principles" , "Prude" , "Adherent ot strict religous principles, opposed to sensual pleasures" , "Over-strict moralist" , "Moralistic type" .)