Intelligent vehicles are on the rise (5)
I believe the answer is:
smart
'intelligent' is the definition.
(associated in meaning)
'vehicles are on the rise' is the wordplay.
'vehicles' becomes 'trams' (tram is a kind of vehicle).
'are on the rise' says the letters should be written backwards (in a down clue, letters go up).
'trams' backwards is 'SMART'.
(Other definitions for smart that I've seen before include "Chic; sting" , "Neat, tidy; sting" , "fly" , "Well-dressed" , "Sort of card" .)