Nissan driver's not completely smooth (4)
I believe the answer is:
sand
'smooth' is the definition.
(I know that sand is a kind of smooth)
'nissan driver's not completely' is the wordplay.
'not completely' indicates a hidden word.
'SAND' is hidden in the letters of 'nissan drivers'.
(Other definitions for sand that I've seen before include "Grit; powdered rock" , "George - - (French authoress)" , "Hourglass particles" , "Granulated quartz" , "Beach susbstance" .)