Test about old city den in books (5,3)
I believe the answer is:
trial run
'test' is the definition.
(I know that test can be written as trial run)
'about old city den in books' is the wordplay.
'about' says the letters should be written in reverse.
'old city' becomes 'ur' (ancient city state).
'den' becomes 'lair' (synonyms).
'in' is an insertion indicator.
'books' becomes 'NT' (abbreviation for New Testament).
'ur'+'lair'='urlair'
'urlair' put into 'nt' is 'nurlairt'.
'nurlairt' in reverse letter order is 'TRIAL RUN'.
(Other definitions for trial run that I've seen before include "Turn rail around in practical test" , "Practice attempt" , "A test for a car" , "Rehearsal" , "Test mission" .)