Immature cow eating the lot (6)
I believe the answer is:
callow
'immature' is the definition.
(similar in meaning)
'cow eating the lot' is the wordplay.
'eating' is an insertion indicator.
'the lot' becomes 'all' (I've seen this before).
'cow' placed around 'all' is 'CALLOW'.
(Other definitions for callow that I've seen before include "lacking experience of life" , "Immature and inexperienced, as youth often is" , "Inexperienced or unsophisticated, as a youth say" , "Jejune" , "Simon ---, actor; immature" .)