Criminal gaoled, it's well established (3,3)
I believe the answer is:
age old
'well established' is the definition.
The answer and definition are different parts of speech. However, adjectives and past participle verbs sometimes define each other.
'criminal gaoled' is the wordplay.
'criminal' is an anagram indicator.
'gaoled' is an anagram of 'AGE-OLD'.
'it's' acts as a link.
Can you help me to learn more?
(Other definitions for age old that I've seen before include "Ancient" , "very traditional" .)