Some Euclidean theory (4)
I believe the answer is:
idea
'theory' is the definition.
(idea can mean a theory)
'some euclidean' is the wordplay.
'some' says the answer is hidden in the clue (some of the parts of the word are used).
'IDEA' is hidden in the letters of 'euclidean'.
(Other definitions for idea that I've seen before include "Mental impression" , "A suspicion of" , "Notion, concept" , "Thought; concept" , "Light bulb above head" .)