Immature beast eating everything (6)
I believe the answer is:
callow
'immature' is the definition.
(similar in meaning)
'beast eating everything' is the wordplay.
'beast' becomes 'cow' (cow is a kind of beast).
'eating' indicates putting letters inside.
'everything' becomes 'all' (all things).
'cow' placed around 'all' is 'CALLOW'.
(Other definitions for callow that I've seen before include "Simon ---, actor; immature" , "Inexperienced or unsophisticated, as a youth say" , "23 [BRIGHTONIAN] actor" , "Green" , "Jejune" .)