Well-behaved, very nearly getting a medal (2,4,2,4)
I believe the answer is:
as good as gold
'well-behaved' is the definition.
(I've seen this before)
'very nearly getting a medal' is the wordplay.
'very nearly' becomes 'as good as' (eg 'as good as dead' means 'almost dead').
'getting a medal' becomes 'gold' (I can't justify this - if you can you should give a lot more credence to this answer).
'asgoodas'+'gold'='AS GOOD AS GOLD'
Can you help me to learn more?
(Other definitions for as good as gold that I've seen before include "very nearly getting medal" , "with perfect manners" , "Giving no trouble at all" , "Really well-behaved" , "Silver is not" .)