Robust car's function drivers primarily treasure (6)
I believe the answer is:
dodgem
'robust car's' is the definition.
'function drivers primarily treasure' is the wordplay.
'function' becomes 'do' (I've seen this in other clues**).
'primarily' indicates taking the first letters.
'treasure' becomes 'gem' ('gem' can be a synonym of 'treasure').
The initial letter of 'drivers' is 'd'.
'do'+'d'+'gem'='DODGEM'
(Other definitions for dodgem that I've seen before include "Fairground amusement vehicle" , "Electric car" , "Bumper car" , "Car used in fairground" , "Small car at funfair for bumping into others" .)