Race small vehicle (6)
I believe the answer is:
strain
'race' is the definition.
(I've seen this before)
'small vehicle' is the wordplay.
'small' becomes 's' (abbreviation - e.g. clothes size).
'vehicle' becomes 'train'.
's'+'train'='STRAIN'
(Other definitions for strain that I've seen before include "Family; effort" , "Make intensive effort" , "Pass through a colander" , "Overwork - melody" , "Pressure - filter" .)