A cereal plant, so we hear, is wrong (4)
I believe the answer is:
awry
'is wrong' is the definition.
(synonyms)
'a cereal plant so we hear' is the wordplay.
'cereal plant' becomes 'rye' (I've seen this before).
'so we hear' shows a homophone (sound like).
'rye' sounds like 'wry'.
'a'+'wry'='AWRY'
(Other definitions for awry that I've seen before include "Crooked, askew" , "Crooked, twisted to one side" , "Twisted; distorted" , "Crooked; amiss" , "the wrong direction" .)