Type of car difficult to better (7)
I believe the answer is:
hardtop
'type of car' is the definition.
(I've seen this before)
'difficult to better' is the wordplay.
'difficult' becomes 'hard' ('hard' can be a synonym of 'difficult').
'to better' becomes 'top' (I've seen this in other clues).
'hard'+'top'='HARDTOP'
(Other definitions for hardtop that I've seen before include "Style of car" , "Rigid-roofed vehicle" , "Type of car with removable roof" , "Car with a rigid top" , "Sort of car" .)