Mischievous child's play resulting in crash (6)
I believe the answer is:
impact
'crash' is the definition.
(I've seen this before)
'mischievous child's play' is the wordplay.
'mischievous child' becomes 'imp' (I've seen this before).
'play' becomes 'act' (synonyms).
'imp'+'act'='IMPACT'
'resulting in' acts as a link.
(Other definitions for impact that I've seen before include "Repercussion" , "Serious effect" , "Impression" , "The striking of one thing against another" , "Force exerted by collision" .)