Vehicle put a strain on electric current (4)
I believe the answer is:
taxi
'vehicle' is the definition.
(I know that taxi is a type of motor car)
'a strain on electric current' is the wordplay.
'a strain' becomes 'tax' (both can mean to overwork or demand a lot from).
'on' means one lot of letters go next to another (in a down clue, letters appear on others).
'electric current' becomes 'I' (physics symbol).
'tax'+'i'='TAXI'
'put' acts as a link.
(Other definitions for taxi that I've seen before include "Vehicle that plies for hire" , "Prepare to fly" , "Hackney carriage" , "(Plane) roll over tarmac" , "Hired vehicle" .)