Dog, good and mature, getting even terrier excited (6,9)
I believe the answer is:
golden retriever
'dog' is the definition.
(golden retriever is a kind of dog)
'good and mature getting even terrier excited' is the wordplay.
'good' becomes 'g' (abbreviation).
'and' says to put letters next to each other.
'mature' becomes 'old' ('old' is associated in meaning with 'mature').
'getting' says to put letters next to each other.
'excited' indicates an anagram.
'even'+'terrier'='eventerrier'
'eventerrier' with letters rearranged gives 'enretriever'.
'g'+'old'+'enretriever'='GOLDEN RETRIEVER'
(Other definitions for golden retriever that I've seen before include "Dog" , "canine" , "pet?" .)