Implant that leads the field (9)
I believe the answer is:
pacemaker
This clue is a double definition.
'implant' is the first definition.
(heart implant)
'that leads the field' is the second definition.
(as in a pacemaker in a race)
(Other definitions for pacemaker that I've seen before include "the first in race" , "leader at the start" , "Implanted device to control the heartbeat" , "Cardiac device" , "Surgical implant" .)