Ornament a lunatic knocked over -- histrionic incident (9)
I believe the answer is:
damascene
'ornament' is the definition.
(damascene is a kind of ornament)
'a lunatic knocked over histrionic incident' is the wordplay.
'lunatic' becomes 'mad' (I've seen this before).
'knocked over' says the letters should be written in reverse.
'histrionic incident' becomes 'scene' (I've seen this before).
'a'+'mad'='amad'
'amad' in reverse letter order is 'dama'.
'dama'+'scene'='DAMASCENE'
(Other definitions for damascene that I've seen before include "of very ancient city" , "Rich ornamentation on metal" , "ornament" , "One from old city" , "Middle Eastern" .)