Immature beast's quite content (6)
I believe the answer is:
callow
'immature' is the definition.
(similar in meaning)
'beast's quite content' is the wordplay.
'beast' becomes 'cow' (cow is a kind of beast).
'quite' becomes 'all' (I've seen this in another clue).
'content' means one lot of letters goes inside another.
'cow' enclosing 'all' is 'CALLOW'.
(Other definitions for callow that I've seen before include "Jejune" , "Simon ---, actor; immature" , "Immature and inexperienced, like youth" , "Immature and inexperienced, as youth often is" , "Inexperienced or unsophisticated, as a youth say" .)