Number diagram (6)
I believe the answer is:
figure
This clue is a double definition.
'number' is the first definition.
(figure is a kind of whole number)
'diagram' is the second definition.
(I've seen this before)
(Other definitions for figure that I've seen before include "Body shape" , "Work out" , "Build" , "Reckon" , "Symbol representing a number" .)