Runner, old, unexpectedly regains lead in race (9)
I believe the answer is:
organiser
'runner' is the definition.
(someone running something)
'old unexpectedly regains lead in race' is the wordplay.
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'unexpectedly' indicates anagramming the letters.
'lead in' indicates taking the first letters.
The initial letter of 'race' is 'r'.
'regains' with letters rearranged gives 'rganise'.
'o'+'rganise'+'r'='ORGANISER'
(Other definitions for organiser that I've seen before include "One managing" , "Manager" , "impresario" , "storage container" , "arranger" .)