So Cryptic crossword clues are puzzles:-
Former Boris favourite rues gay scandal. (3,4)
One end of the clue is a direct clue to the answer. The rest is a set of instructions as to how to solve it.
In this case the "definition" is "Former Boris favourite" - a sarcastic reference to Sue grey and her investigation of Borisgate.
scandal I think is an anagram indicator. If we scandalise "rues gay" ) i.e. mix it up - we get "Sue Grey".