Person taking the lead (5)
I believe the answer is:
pacer
'person taking the lead' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for pacer that I've seen before include "Racing horse with an unusual gait" , "setting the rate" , "One who sets the running speed; horse" , "One setting race speed" , "Racehorse" .)