Improvement concerning structure (6)
I believe the answer is:
reform
'improvement' is the definition.
(I know that reform is a type of improvement)
'concerning structure' is the wordplay.
'concerning' becomes 're' (both can mean on a particular topic).
'structure' becomes 'form' (form is a kind of structure).
're'+'form'='REFORM'
(Other definitions for reform that I've seen before include "Show improvement" , "Change one's ways (for the better?)" , "Abandon evil practices" , "Go straight" , "Mend one's ways" .)