Order I'm to send back for cereal grass (6)
I believe the answer is:
millet
'cereal grass' is the definition.
(millet is a kind of cereal grass)
'order i'm to send back' is the wordplay.
'order' becomes 'tell' (synonyms).
'i'm' becomes 'im' ().
'to send back' shows that the letters should be reversed in order (I've seen 'sends back' mean this).
'tell'+'im'='tellim'
'tellim' written backwards gives 'MILLET'.
'for' acts as a link.
(Other definitions for millet that I've seen before include "Cereal grass for the French painter" , "Type of grain" , "Cereal; French artist" , "Food-grain" , "sort of grass" .)