Corner at 90? (5,5)
I believe the answer is:
right angle
'corner at 90?' is the definition.
(90 degree angle)
'corner at 90?' is the wordplay.
'corner' becomes 'angle' (eg a corner of a shape).
'at' says to put letters next to each other (I've seen this in other clues).
'90?' becomes 'right' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'angle' put after 'right' is 'RIGHT-ANGLE'.
Can you help me to learn more?
(Other definitions for right angle that I've seen before include "Part of a square" , "a good number of degrees?" , "that's formed by 1 Across and Down" , "Ninety-degree corner" , "Ninety degrees" .)