Geometric theory eccentric uncle maintains (9)
I believe the answer is:
euclidean
'geometric' is the definition.
Both the definition and answer are adjectives. Perhaps there's an association between them I don't understand?
'theory eccentric uncle maintains' is the wordplay.
'theory' becomes 'idea' (idea can mean a theory).
'eccentric' indicates an anagram.
'maintains' is an insertion indicator.
'uncle' with letters rearranged gives 'eucln'.
'idea' inserted inside 'eucln' is 'EUCLIDEAN'.
Can you help me to learn more?
(Other definitions for euclidean that I've seen before include "kind of geometry" , "Of an ancient system of geometry" , "of ancient Greek" , "type of geometry" .)