Jockey's dire, thrown before start of race (5)
I believe the answer is:
rider
'jockey's' is the definition.
(thesaurus)
'dire thrown before start of race' is the wordplay.
'thrown' indicates an anagram.
'before' means one lot of letters go next to another.
'start of' indicates taking the first letters.
The initial letter of 'race' is 'r'.
'dire' anagrammed gives 'ride'.
'ride'+'r'='RIDER'
(Other definitions for rider that I've seen before include "Equestrian addition by a jury" , "Horse(wo)man" , "Supplementary verdict brought in by jury" , "Comment from jury" , "Clause appended to a legislative bill" .)