Nasty slime covering universal breakfast dish (6)
I believe the answer is:
muesli
'breakfast dish' is the definition.
(I've seen this before)
'nasty slime covering universal' is the wordplay.
'nasty' is an anagram indicator.
'covering' indicates putting letters inside.
'universal' becomes 'u'.
'slime' anagrammed gives 'mesli'.
'mesli' enclosing 'u' is 'MUESLI'.
(Other definitions for muesli that I've seen before include "Breakfast mixture of oats, nuts, fruit etc" , "Breakfast cereal mix" , "Mule is fed with breakfast compound" , "Mixture of cereals etc" , "Type of breakfast cereal" .)