Deface an old penny displayed in case (9)
I believe the answer is:
vandalise
'deface' is the definition.
(I know that deface is a more specific form of the action vandalise)
'an old penny displayed in case' is the wordplay.
'old penny' becomes 'd' (abbreviation for the pre-decimal penny).
'displayed in' indicates putting letters inside.
'case' becomes 'valise' (valise is a kind of case).
'an'+'d'='and'
'and' inserted within 'valise' is 'VANDALISE'.
(Other definitions for vandalise that I've seen before include "Deliberately damage" , "Inflict malicious damage" , "trash" , "Wreck" , "Wilfully damage" .)