Shapes up, primarily in relation to corners (9)
I believe the answer is:
triangles
'shapes' is the definition.
(triangle is a kind of shape)
'up primarily in relation to corners' is the wordplay.
'up' says the letters should be written in reverse (in down clue: letters go upwards).
'primarily' suggests taking the first letters.
'corners' becomes 'angles' (I've seen this before).
The first letters of 'in relation to' is 'irt'.
'irt' backwards is 'tri'.
'tri'+'angles'='TRIANGLES'
(Other definitions for triangles that I've seen before include "Geometrical figures" , "Three-sided figures" , "Three-sided shapes" , "Percussion instruments" , "Three-sided polygons" .)