Having all sides of the same length (11)
I believe the answer is:
equilateral
'having all sides of the same length' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for equilateral that I've seen before include "having every side the same length" , "square, perhaps" , "keeping the sides the same" , "like some figures on plane" , "(Of a triangle) having sides the same" .)