Eat extremely hot cereal (not cold!). It's out of this world (8)
I believe the answer is:
ethereal
'eat extremely hot' is the definition.
I can't judge whether this defines the answer.
'cereal not cold it's out of this world' is the wordplay.
'not' is a deletion indicator.
'cold' becomes 'c' (eg on taps).
'it's out of' means one lot of letters goes inside another (some letters on the outside of others).
'this world' becomes 'the' (I am not sure about this - if you are sure you should believe this answer much more).
'cereal' with 'c' removed is 'ereal'.
'ereal' placed around 'the' is 'ETHEREAL'.
Can you help me to learn more?
(Other definitions for ethereal that I've seen before include "Of the spirit" , "Celestial; insubstantial" , "There in lea - it looks delicate" , "Delicate - airy" , "exquisitely airy" .)