Bordering a hypotenuse? (9)
I believe the answer is:
alongside
'bordering' is the definition.
(I've seen this before)
I can't explain the rest of the clue.
(Other definitions for alongside that I've seen before include "Aligned so as to be next to, as ship moored to pier" , "Bordering" , "Side by side" , "Lying next to, as a boat might be" , "Age-old sin (anag.)" .)