Shock performance by monkey (6)
I believe the answer is:
impact
'shock' is the definition.
(thesaurus)
'performance by monkey' is the wordplay.
'performance' becomes 'act' (act is a kind of performance).
'by' is a charade indicator (letters next to each other).
'monkey' becomes 'imp' ('imp' can be a synonym of 'monkey').
'act' put after 'imp' is 'IMPACT'.
(Other definitions for impact that I've seen before include "Bear upon" , "Forcible contact; marked influence" , "Serious effect" , "Impression" , "Collision - effect" .)