Diagram to be understandable (6)
I believe the answer is:
figure
I believe this clue is a double definition.
'diagram' is the first definition.
(I've seen this before)
'to be understandable' is the second definition.
(as in phrases like 'that figures')
(Other definitions for figure that I've seen before include "Symbol representing a number" , "Work out" , "Shape; number" , "form" , "Come to understand - shape" .)