Cereal crop I planted in jungle (5)
I believe the answer is:
maize
'cereal crop' is the definition.
(I've seen this before)
'i planted in jungle' is the wordplay.
'planted in' means one lot of letters goes inside another.
'jungle' becomes 'maze' (I've seen this in other clues).
'i' inserted into 'maze' is 'MAIZE'.
(Other definitions for maize that I've seen before include "Cereal crop" , "- maybe staple" , "source of food" , "Tall cereal grass producing large edible ears" , "American corn" .)