Go when penultimate lap ends in race (6)
I believe the answer is:
depart
'go' is the definition.
('depart' can be a synonym of 'go')
'penultimate lap ends in race' is the wordplay.
'ends' suggests the final letters.
'in' is an insertion indicator.
'race' becomes 'dart' (darting is a kind of racing**).
The final letters of 'penultimate lap' are 'ep'.
'ep' inserted inside 'dart' is 'DEPART'.
'when' acts as a link.
(Other definitions for depart that I've seen before include "Retire from" , "Leave, vanish" , "Clearly expressive" , "Go away or leave" , "Prated (anag.)" .)