Everything eaten by ruminant is green (6)
I believe the answer is:
callow
'is green' is the definition.
(I've seen this before)
'everything eaten by ruminant' is the wordplay.
'everything' becomes 'all' (all things).
'eaten by' means one lot of letters goes inside another.
'ruminant' becomes 'cow' (cow is a kind of ruminant).
'all' put inside 'cow' is 'CALLOW'.
(Other definitions for callow that I've seen before include "Inexperienced or innocent" , "Young and inexperienced" , "Simon ---, actor; immature" , "Jejune" , "lacking experience of life" .)