### Clue: Collision has traffic, initially better on motorway, turned back (6)
### Answer: IMPACT 
 ### Explanation: Definition: Collision
 **Reversal of**(…, turned back) [ **1st letter of**(traffic, initially) "" + CAP(to better/to outdo) **plus**(on) MI(the English motorway, M1 with the Roman numeral substitution) ].