Breakfast mixture (7)
'breakfast mixture' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for muesli that I've seen before include "Breakfast food of cereal, nuts, dried fruit etc with milk" , "Breakfast food of cereals, dried fruit, nuts etc" , "Swiss breakfast?" , "Granola-like food" , "Mule is fed with breakfast compound" .)
(Other definitions for granola that I've seen before include "US breakfast cereal" , "breakfast dish" , "type of cereal" .)