Sphere, yes; but it's often rectangular (5)
I believe the answer is:
field
'sphere' is the definition.
(I know that sphere can be written as field)
I don't understand how the remainder of the clue works.
Can you help me to learn more?
(Other definitions for field that I've seen before include "Crop land; catch (ball)" , "Games area" , "his nocturnes, perhaps?" , "Pitch" , "Enclosed piece of land or area of endeavour" .)