A time trial witness (6)
I believe the answer is:
attest
'witness' is the definition.
(thesaurus)
'a time trial' is the wordplay.
'time' becomes 't'.
'trial' becomes 'test' ('test' can be a synonym of 'trial').
'a'+'t'+'test'='ATTEST'
(Other definitions for attest that I've seen before include "Demonstrate" , "Vouch for" , "Offer evidence" , "Verify" , "show?" .)