Sixth book of the Old Testament (6)
I believe the answer is:
joshua
'sixth book of the old testament' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for joshua that I've seen before include "He led the Israelites after the death of Moses" , "Fighter at Jericho" , "Sixth book of the Old Testament" , "Conqueror of Jericho" , "Biblical character" .)