Girl in lead circling till race's conclusion (6)
I believe the answer is:
phoebe
'girl' is the definition.
'lead circling till race's conclusion' is the wordplay.
'lead' becomes 'pb' (Pb is the chemical symbol for lead**).
'circling' indicates putting letters inside (circling can mean going around or containing).
'till' becomes 'hoe' (hoeing is a kind of tilling).
'conclusion' indicates one should take the final letters.
The final letter of 'race' is 'e'.
'pb' placed around 'hoe' is 'phoeb'.
'phoeb'+'e'='PHOEBE'
'in' is the link.
(Other definitions for phoebe that I've seen before include "Old poetic name for moon, and satellite of Saturn" , "Apollo's sister Artemis, the moon goddess" , ""moony" woman" , "Daughter of Uranus and Gaia" , "Titan, daughter of Uranus -- satellite of Saturn" .)