Mark, when cycling, gets a crusty dish (6)
I believe the answer is:
gratin
'crusty dish' is the definition.
I don't understand how the rest of the clue works.
(Other definitions for gratin that I've seen before include "Dish covered with grated cheese" , "A dish cooked with grated cheese on it" , "'Au ..., with a cheesy topping (6)'" , "With a crisp, golden topping" , "Au . . . . . ., cooked with a covering of cheese" .)