Class structure (4)
I believe the answer is:
form
I believe this clue is a double definition.
'class' is the first definition.
('form' can be a synonym of 'class')
'structure' is the second definition.
(form is a kind of structure)
(Other definitions for form that I've seen before include "Acceptable conduct" , "Class in a school, or the shape of something" , "Make" , "Hare's home - something to be filled in" , "Configuration; type" .)