Clue: Test horse going round track before start of race (3,3)
Answer: DRY RUN
Explanation: Definition: Test
 DUN(a horse with a particular colouring) **containing**(going round) [ RY(abbrev. for "railway", which runs on tracks/rails) **placed above**(before, in a down clue) **1st letter of**(start of) "race" ].
Defn: …/a rehearsal.