From the blog (credit to Goujeers) “Alec Robins (formerly Custos in the Guardian, Zander in the Listener) in Teach Yourself Crosswords (reissued as the ABC of Crosswords) suggested borrowing the term “cook” from chess puzzles, where it is used for a correct answer that was not the one intended.”
The clue in question was…
25 Is highly moving in theatre part (5)
… a double definition which could be answered as FLIES (intended) or WINGS (not intended and therefore a “cook”)