Frontrunners of endurance race aim for great time (3)
I believe the answer is:
era
'great time' is the definition.
(I've seen this in another clue)
'frontrunners of endurance race aim' is the wordplay.
'frontrunners of' indicates taking the first letters.
The initial letters of 'endurance race aim' is 'ERA'.
'for' acts as a link.
(Other definitions for era that I've seen before include "Age of importance" , "US equality law" , "Distinctive period of time" , "Long historical period" , "Are involved in time span" .)