Secondary selection of vehicles serviced (6)
I believe the answer is:
lesser
'secondary' is the definition.
(secondary can mean of lesser importance)
'selection of vehicles serviced' is the wordplay.
'selection of' indicates the answer is hidden within the clue.
'LESSER' is hidden in the letters of 'vehicles serviced'.
(Other definitions for lesser that I've seen before include "Smaller; inferior" , "Smaller in size or amount or value" , "Not as great" , "That which is smaller, as of two evils" , "Smaller, minor" .)