Racing driver behind then ahead (6)
I believe the answer is:
button
'racing driver' is the definition.
(F1 driver Jenson Button)
'behind then ahead' is the wordplay.
'behind' becomes 'butt' (both can mean the buttocks).
'then' is a charade indicator (letters next to each other).
'ahead' becomes 'on' (synonyms).
'butt'+'on'='BUTTON'
(Other definitions for button that I've seen before include "Garment fastener" , "Disc for pushing, fastening" , "On-screen control" , "Fastener to fit through a hole" , "Fastener - English F1 racing driver" .)