Historian Sandy's crushed by vehicle (7)
I believe the answer is:
carlyle
'historian' is the definition.
(Thomas Carlyle)
'sandy's crushed by vehicle' is the wordplay.
'sandy's' becomes 'lyle' (I can't explain this - if you can you should give a lot more credence to this answer).
'crushed by' means one lot of letters go next to another (in a down clue, some letters go on top of others).
'vehicle' becomes 'car' (car is a kind of vehicle).
'lyle' put after 'car' is 'CARLYLE'.
Can you help me to learn more?
(Other definitions for carlyle that I've seen before include "Scottish writer" , "Scots historian" , "Victorian historian" , "Thomas --, Scottish social historian" , "Thomas -, nineteenth century British essayist/historian" .)