Miles sick eating up first cereal (6)
I believe the answer is:
muesli
'cereal' is the definition.
(I know that muesli is a type of cereal)
'miles sick eating up first' is the wordplay.
'sick' is an anagram indicator.
'eating' means one lot of letters goes inside another.
'up first' becomes 'u' (1st letter of 'up').
'miles' is an anagram of 'mesli'.
'mesli' going around 'u' is 'MUESLI'.
(Other definitions for muesli that I've seen before include "Breakfast mush" , "Swiss cereal" , "Breakfast dish of cereal, dried fruit and nuts in milk" , "Cereal made with oats and nuts" , "Rolled oats, nuts and fruit mixture" .)