US star has cereal mostly — or fruit (6)
I believe the answer is:
cherry
'fruit' is the definition.
(cherry is a kind of fruit)
'us star has cereal mostly' is the wordplay.
'us star' becomes 'cher' (I can't explain this - if you can you should believe this answer much more).
'has' means one lot of letters go next to another.
'cereal' becomes 'rye' (rye is a kind of cereal).
'mostly' means to remove the last letter (most but not all of the word).
'rye' with its last letter taken away is 'ry'.
'cher'+'ry'='CHERRY'
'or' acts as a link.
Can you help me to learn more?
(Other definitions for cherry that I've seen before include "Greyfriars Bob" , "Cricket ball" , "Tree; red fruit" , "Tree with stoned fruit" , "one likely to be picked?" .)