Some Euclidean concept (4)
I believe the answer is:
idea
'concept' is the definition.
(I've seen this in another clue)
'some euclidean' is the wordplay.
'some' says the answer is hidden in the clue (some of the parts of the word are used).
'IDEA' can be found hidden inside 'euclidean'.
(Other definitions for idea that I've seen before include "a wheeze" , "Theory" , "Content of cognition" , "Inkling" , "Mental impression" .)