Witness with last word in trial time (9)
I believe the answer is:
testament
'witness' is the definition.
(I've seen this before)
'last word in trial time' is the wordplay.
'last word' becomes 'amen' (I've seen this before).
'in' means one lot of letters goes inside another.
'trial' becomes 'test' (synonyms).
'time' becomes 't' (abbreviation).
'test'+'t'='testt'
'amen' placed within 'testt' is 'TESTAMENT'.
'with' acts as a link.
(Other definitions for testament that I've seen before include "Earnest" , "Statement about final intentions" , "(Person's) will" , "Witness" , "with 1a [NEW] where is Mark?" .)