This is a plum site the cryptic clue for which underground station in London?

The word "plum" is a slang for "very good". The clue "underground station in London" indicates that we are looking for a name of an underground station. The word "Sloane" is a homophone of "slon", which is a Polish word for "elephant". The word "Square" is a homophone of "skwer", which is a Polish word for "park". So the cryptic clue "This is a plum site the cryptic clue for which underground station in London?" is a clue for the underground station Sloane Square.