A cereal heard to surprise (5)
I believe the answer is:
amaze
'surprise' is the definition.
(I know that amaze is a more specific form of the action surprise)
'a cereal heard' is the wordplay.
'cereal' becomes 'maize' (maize is a kind of cereal).
'heard' shows a homophone (sound like).
'maize' is a homophone of 'maze'.
'a'+'maze'='AMAZE'
'to' acts as a link.
(Other definitions for amaze that I've seen before include "Bewilder" , "wow!" , "Stun" , "Floor" , "baffle" .)