Injured party starts to video it, Chelsea tractor in motion (6)
I believe the answer is:
victim
'injured party' is the definition.
(I've seen this before)
'starts to video it chelsea tractor in motion' is the wordplay.
'starts to' suggests taking the first letters.
The initial letters of 'video it chelsea tractor in motion' is 'VICTIM'.
(Other definitions for victim that I've seen before include "Object of attack" , "One suffers from adverse circumstances" , "Fall guy" , "Casualty of a crime" , "Person injured or killed by another" .)