Turn on American runner after start of race (6)
I believe the answer is:
arouse
'turn on' is the definition.
(I've seen this before)
'american runner after start of race' is the wordplay.
'american' becomes 'a' (common abbreviation - e.g. in organisation names).
'runner' becomes 'ouse' (Yorkshire river**).
'after' is a charade indicator (letters next to each other).
'start of' says to take the initial letters.
The initial letter of 'race' is 'r'.
'ouse' put after 'r' is 'rouse'.
'a'+'rouse'='AROUSE'
(Other definitions for arouse that I've seen before include "Awake from sleep" , "Waken up or stimulate feeling" , "Turn on" , "Incite or stimulate" , "Evoke (a feeling)" .)