Longest side of a right-angled triangle (10)
I believe the answer is:
hypotenuse
'longest side of a right-angled triangle' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for hypotenuse that I've seen before include "Side opposite right angle" , "geometrician's line" , "Longest side of right-angled triangle" , "long line" , "Long side of right-angled triangle" .)