Castigation about a demonstration (7)
I believe the answer is:
reproof
'castigation' is the definition.
(I've seen this before)
'about a demonstration' is the wordplay.
'about' becomes 're' ('re' means regarding or about).
'a demonstration' becomes 'proof' (I've seen this before).
're'+'proof'='REPROOF'
(Other definitions for reproof that I've seen before include "Reprimand" , "Dressing down" , "what mac no longer keeping one dry needs?" , "Rebuke" , "Ticking off" .)