Mystery driver and short friend getting A marks (8)
I believe the answer is:
stigmata
'marks' is the definition.
(plural of stigma - stigma can mean a mark or wound)
'mystery driver and short friend getting a' is the wordplay.
'mystery driver' becomes 'Stig' (the Stig is a driver in the TV show Top Gear).
'and' says to put letters next to each other.
'short' means to remove the last letter.
'friend' becomes 'mate' (mate is a kind of friend).
'getting' is a charade indicator (letters next to each other).
'mate' with its final letter taken away is 'mat'.
'stig'+'mat'+'a'='STIGMATA'
(Other definitions for stigmata that I've seen before include "Christ's marks of crucifixion" , "Marks resembling the wounds of Christ" , "Disgraces" , "Marks corresponding to the wounds on Christ's body" , "Mercian king who built dyke" .)