Colt running miles in an unspecified course (8)
I believe the answer is:
consomme
'course' is the definition.
(I've seen this before)
'colt running miles in an unspecified' is the wordplay.
'colt' becomes 'c' (horseracing abbreviation).
'running' becomes 'on' (eg when a machine is running it is on).
'miles' becomes 'm' (common abbreviation).
'in' indicates putting letters inside.
'an unspecified' becomes 'some' (some unspecified thing).
'm' placed within 'some' is 'somme'.
'c'+'on'+'somme'='CONSOMME'
(Other definitions for consomme that I've seen before include "Clear initial course" , "Liquid food" , "Clear French meat soup" , "Thin, clear soup" , "stock product" .)