Terribly slim, consuming ends of uneatable cereal (6)
I believe the answer is:
muesli
'cereal' is the definition.
(I've seen this before)
'terribly slim consuming ends of uneatable' is the wordplay.
'terribly' indicates an anagram.
'consuming' indicates putting letters inside.
'ends of' means to remove the middle letters.
'uneatable' with its centre removed is 'ue'.
'slim' is an anagram of 'msli'.
'msli' enclosing 'ue' is 'MUESLI'.
(Other definitions for muesli that I've seen before include "Granola-like food" , "Mixture of crushed cereals. nuts and fruit" , "Breakfast mixture of dry cereals and fruits" , "Breakfast food of cereals, fruits, nuts etc" , "Mixture of crushed grain, nuts etc" .)