Part of a stable diet (3)
I believe the answer is:
hay
'part of a stable diet' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for hay that I've seen before include "Meadow crop" , "Make . . . while the sun shines" , "Animal fodder" , "basic bedding" , "Wye book town" .)