Fast driver great in two runs (5)
I believe the answer is:
racer
'fast driver' is the definition.
(I've seen this before)
'great in two runs' is the wordplay.
'great' becomes 'ace' (ace can informally mean very good).
'in' means one lot of letters goes inside another.
'two' indicates the doubling of some letters.
'runs' becomes 'r' (cricket abbreviation).
'r' duplicated is 'rr'.
'ace' going within 'rr' is 'RACER'.
(Other definitions for racer that I've seen before include "Greyhound" , "Fast competitive car or animal" , "Fast-moving snake" , "Fast-moving animal" , "sportsman" .)