Witness's source of tension during a trial (6)
I believe the answer is:
attest
'witness's' is the definition.
(thesaurus)
'source of tension during a trial' is the wordplay.
'source of' indicates taking the first letters.
'during' indicates putting letters inside.
'trial' becomes 'test' ('test' can be a synonym of 'trial').
The initial letter of 'tension' is 't'.
'a'+'test'='atest'
't' going within 'atest' is 'ATTEST'.
(Other definitions for attest that I've seen before include "Bear formal witness to, give evidence" , "Act as witness" , "say, in court" , "Give evidence in a court of law" , "Vouch for" .)