Electrical device in vehicles, one not used earlier (11)
I believe the answer is:
transformer
'electrical device' is the definition.
(transformer is a kind of electrical device)
'vehicles one not used earlier' is the wordplay.
'vehicles' becomes 'trains'.
'one' becomes 'i' (Roman numeral).
'not used' suggests deleting specific letters.
'earlier' becomes 'former'.
'trains' with 'i' taken away is 'trans'.
'trans'+'former'='TRANSFORMER'
'in' acts as a link.
(Other definitions for transformer that I've seen before include "Electrical device used to change voltage" , "Electrical voltage changer" , "piece of electrical equipment" , "Device for changing voltage" , "Voltage-changing device" .)