A race established so you can bear witness (6)
I believe the answer is:
attest
'bear witness' is the definition.
(I know that bear witness can be written as attest)
'a race established' is the wordplay.
'race' becomes 'tt'.
'established' becomes 'est' (abbreviation).
'a'+'tt'+'est'='ATTEST'
'so you can' acts as a link.
This may not be correct. Some or all of it may belong to another bit of the clue.
Can you help me to learn more?
(Other definitions for attest that I've seen before include "Give testimony in a court of law" , "Provide firm evidence" , "Corroborate" , "Bear formal witness to, give evidence" , "Declare to be true or genuine" .)