Admonishment about demonstration (7)
I believe the answer is:
reproof
'admonishment' is the definition.
(thesaurus)
'about demonstration' is the wordplay.
'about' becomes 're' (regarding).
'demonstration' becomes 'proof' (I've seen this before).
're'+'proof'='REPROOF'
(Other definitions for reproof that I've seen before include "Ticking-off" , "Scolding" , "Castigation" , "Rebuke" , "work on raincoat?" .)