A trial involving tense witness (6)
I believe the answer is:
attest
'witness' is the definition.
(I've seen this before)
'a trial involving tense' is the wordplay.
'trial' becomes 'test' (synonyms).
'involving' indicates putting letters inside.
'tense' becomes 't' (abbreviation used in many dictionaries).
'test' enclosing 't' is 'ttest'.
'a'+'ttest'='ATTEST'
(Other definitions for attest that I've seen before include "Declare to be true or genuine" , "Adjure" , "show?" , "Verify" , "Give evidence - sounds a trial" .)