Collision involving mischievous child before performance (6)
I believe the answer is:
impact
'collision' is the definition.
(I've seen this in another clue)
'mischievous child before performance' is the wordplay.
'mischievous child' becomes 'imp' (imp can mean a misbehaving child).
'before' says to put letters next to each other.
'performance' becomes 'act' (act is a kind of performance).
'imp'+'act'='IMPACT'
'involving' is the link.
(Other definitions for impact that I've seen before include "The force fo violent strike" , "Forceful contact between two objects" , "Force of blow or collision" , "Forcible contact; marked influence" , "The force of one body against another" .)