Almost grand and mature, very well-behaved (2,4,2,4)
I believe the answer is:
as good as gold
'very well-behaved' is the definition.
(I've seen this before)
'almost grand and mature' is the wordplay.
'almost' becomes 'asgoodas' (I am not sure about this - if you are sure you should believe this answer much more).
'grand' becomes 'g' (G is short for grand as in a thousand).
'and' says to put letters next to each other.
'mature' becomes 'old' (associated in meaning).
'asgoodas'+'g'+'old'='AS GOOD AS GOLD'
Can you help me to learn more?
(Other definitions for as good as gold that I've seen before include "Perfectly behaved" , "Giving no trouble at all" , "Behaving very well" , "very nearly getting medal" , "Silver is not" .)