As a cereal, it's what goes round the earth (5)
I believe the answer is:
wheat
'as a cereal' is the definition.
I can't tell whether this defines the answer.
'what goes round the earth' is the wordplay.
'goes round' indicates putting letters inside.
'the earth' becomes 'E'.
'what' placed around 'e' is 'WHEAT'.
'it's' is the link.
Can you help me to learn more?
(Other definitions for wheat that I've seen before include "Annual grass with light brown grains" , "Cereal plant" , "Type of cereal" , "Tall grass producing light brown grain" , "Cereal grain" .)