Send a letter of request as result of vehicle collision (5-3)
I believe the answer is:
write-off
I believe this clue is a double definition.
'send a letter of request' is the first definition.
(as in writing off to order something)
'result of vehicle collision' is the second definition.
(as in a car being written off)
'as' is the link.
(Other definitions for write-off that I've seen before include "Send a letter - complete failure" , "Car too costly to repair" , "Cancel, a debt perhaps" , "Article beyond repair" , "Destroy (car)" .)