Censure about mathematical demonstration (7)
I believe the answer is:
reproof
'censure' is the definition.
(I've seen this before)
'about mathematical demonstration' is the wordplay.
'about' becomes 're' (regarding).
'mathematical demonstration' becomes 'proof' (proving something mathematically).
're'+'proof'='REPROOF'
(Other definitions for reproof that I've seen before include "Castigation" , "Once more get water-resistant" , "blame" , "Blaming" , "Reprimand" .)