I recently discovered the wonderful Guardian Crossword Blog and there I read this astonishing 1982 tour-de-force from the legendary Guardian crossword setter “Araucaria”. The clue was
O hark the herald angels sing the Boy’s descent which lifted up the world (anag), and in what circumstances (5,9,7,5,6,2,5,3,6,2,3,6)
The solution occupied 9 of the down lights (See 2, See 2 etc.) and may be the longest anagram ever to appear in a mainstream crossword puzzle. Hats off to Araucaria!