Vehicles covered by extra insurance (6)
I believe the answer is:
trains
'vehicles' is the definition.
'covered by extra insurance' is the wordplay.
'covered by' says the answer is hidden in the clue.
'TRAINS' can be found hidden inside 'extra insurance'.
(Other definitions for trains that I've seen before include "Sets of railway cars" , "Prepares by practice for performance" , "follows exercise regime" , "Public transport vehicles with carriages" , "Works out" .)