Cereal is not finished in the labyrinth (5)
I believe the answer is:
maize
'cereal' is the definition.
(maize is a kind of cereal)
'is not finished in the labyrinth' is the wordplay.
'not finished' means to remove the last letter.
'in the' means one lot of letters goes inside another.
'labyrinth' becomes 'maze' (synonyms).
'is' with its last letter taken away is 'i'.
'i' put into 'maze' is 'MAIZE'.
(Other definitions for maize that I've seen before include "one could make a meal of it" , "Oil producer" , "Plant producing sweetcorn" , "grain crop" , "plant with ears" .)