Girl in early race -- that's the girl? (7)
I believe the answer is:
heather
'girl' is the definition.
'early race that's the girl?' is the wordplay.
'early race' becomes 'heat'.
'that's the girl?' becomes 'her'.
'heat'+'her'='HEATHER'
'in' acts as a link.
(Other definitions for heather that I've seen before include "Girl" , "Moorland shrub" , "Low shrub; mixed-colour (yarn)" , "Girl's name - ling" , "that grows up in the Highlands" .)