Cereal grass forming part of military experiment (3)
I believe the answer is:
rye
'cereal grass' is the definition.
(I know that rye is a type of cereal grass)
'part of military experiment' is the wordplay.
'part of' says the answer is hidden in the clue.
'RYE' is hidden within 'military experiment'.
'forming' is the link.
(Other definitions for rye that I've seen before include "Cereal - liquor" , "J D Salinger novel 'The Catcher in the ...'" , "Bitterly regret" , "American whisky" , "Whisky; Sussex town" .)