Very nearly trained large dog to be well behaved (2,4,2,4)
I believe the answer is:
as good as gold
'be well behaved' is the definition.
(as good as gold means very well-behaved)
'very nearly trained large dog' is the wordplay.
'very nearly' becomes 'as good as' (eg 'as good as dead' means 'almost dead').
'trained' indicates anagramming the letters (to train can mean to grow something into a shape).
'large' becomes 'L' (eg in clothes sizes).
'l'+'dog'='ldog'
'ldog' anagrammed gives 'gold'.
'asgoodas'+'gold'='AS GOOD AS GOLD'
'to' acts as a link.
(Other definitions for as good as gold that I've seen before include "(Of children) behaving in an exemplary manner" , "Giving no trouble at all" , "very nearly getting medal" , "Really well-behaved" , "Behaving very well" .)