A turn in the direction of an American car (4)
I believe the answer is:
auto
'an american car' is the definition.
(I've seen this before)
'a turn in the direction of' is the wordplay.
'turn' becomes 'u' (a 'U-turn' is a kind of turn).
'in the direction of' becomes 'to'.
'a'+'u'+'to'='AUTO'
(Other definitions for auto that I've seen before include "Prefix meaning self or same" , "Motor vehicle, in short" , "Prefix denoting by or of the same person or thing" , "Lincoln?" , "Car (American)" .)