Fruit with a cereal (5)
I believe the answer is:
acorn
'fruit' is the definition.
(acorn is a kind of fruit)
'with a cereal' is the wordplay.
'with' says to put letters next to each other.
'cereal' becomes 'corn' (corn is a kind of cereal**).
'a' put next to 'corn' is 'ACORN'.
(Other definitions for acorn that I've seen before include "Nut of oak tree" , "From which a tall tree may grow" , "Tree seed" , "cup's content" , "Potential oak" .)