A car, one nearly new, overturning? It’s a mystery (6)
I believe the answer is:
enigma
'a mystery' is the definition.
(I know that mystery can be written as enigma)
'a car one nearly new overturning?' is the wordplay.
'car' becomes 'mg' ().
'one' becomes 'I' (Roman numeral).
'nearly' means to remove the last letter.
'overturning?' is a reversal indicator (I've seen 'overturning' mean this).
'new' with its final letter removed is 'ne'.
'a'+'mg'+'i'+'ne'='amgine'
'amgine' backwards is 'ENIGMA'.
'it's' acts as a link.
(Other definitions for enigma that I've seen before include "Difficult problem" , "cryptic puzzle!" , "Riddle - German coding machine" , "German military cipher machine" , "Puzzle of gamine (6)," .)