Whole testament, account in it (6)
I believe the answer is:
intact
'whole' is the definition.
(similar in meaning)
'testament account in it' is the wordplay.
'testament' becomes 'nt'.
'account' becomes 'ac' (abbreviation).
'in' is an insertion indicator.
'nt'+'ac'='ntac'
'ntac' put within 'it' is 'INTACT'.
(Other definitions for intact that I've seen before include "without flaw?" , "Perfect" , "Undiminished" , "Inviolate" , "Whole and complete" .)