A driver's first crack off course (6)
I believe the answer is:
adrift
'off course' is the definition.
(adrift can mean off course or astray)
'a driver's first crack' is the wordplay.
'driver's first' becomes 'd' (rst letter of 'driver').
'crack' becomes 'rift' (rift is a kind of crack).
'a'+'d'+'rift'='ADRIFT'
(Other definitions for adrift that I've seen before include "Aimlessly floating" , "Going out of control" , "without purpose or direction" , "Off course" , "Unanchored at sea" .)