Tough, best car (7)
I believe the answer is:
hardtop
'car' is the definition.
(hardtop is a kind of car)
'tough best' is the wordplay.
'tough' becomes 'hard' (synonyms).
'best' becomes 'top' (top can mean the very best).
'hard'+'top'='HARDTOP'
(Other definitions for hardtop that I've seen before include "Rigid-roofed vehicle" , "Non-convertible car" , "Sort of car" , "Style of car" , "Car with a rigid top" .)