Doughnut, mathematically (5)
I believe the answer is:
torus
'doughnut mathematically' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for torus that I've seen before include "Shape of doughnut" , "Architectural moulding" , "Ring shaped like doughnut with hole" , "Ring-doughnut shape" , "Ring-shaped object" .)