They try an Old Testament book (6)
I believe the answer is:
judges
I believe this is a double definition.
'they try' is the first definition.
(I've seen this before)
'an old testament book' is the second definition.
(I've seen this before)
(Other definitions for judges that I've seen before include "holy book" , "Book after Joshua" , "OT book" , "Deems" , "Decides (results of competition)" .)