Appearing placid is good in unusually noisy age (4-5)
I believe the answer is:
easy-going
'appearing placid' is the definition.
(easy-going means relaxed or placid)
'good in unusually noisy age' is the wordplay.
'good' becomes 'g' (abbreviation).
'in' means one lot of letters goes inside another.
'unusually' is an anagram indicator.
'noisy'+'age'='noisyage'
'noisyage' is an anagram of 'easyoing'.
'g' inserted into 'easyoing' is 'EASY-GOING'.
'is' is the link.
(Other definitions for easy-going that I've seen before include "Carefree - relaxed" , "Happy-go-lucky" , "Calm - relaxed - nonchalant" , "Placid; over-tolerant" , "Placid and tolerant, relaxed personality" .)