Cereal plant royalty oddly ignored (3)
I believe the answer is:
oat
'cereal plant' is the definition.
(I've seen this before)
'royalty oddly ignored' is the wordplay.
'oddly ignored' indicates alternate letters.
The alternating letters of 'royalty' are 'OAT'.
(Other definitions for oat that I've seen before include "Cereal grass" , "Cereal (singular)" , "Food grain" , "Basis of porridge" , "Cereal plant" .)