Weapon tried out on book (7)
I believe the answer is:
trident
'weapon' is the definition.
(trident is a kind of weapon)
'tried out on book' is the wordplay.
'out' is an anagram indicator (out can mean wrong or inaccurate).
'on' means one lot of letters go next to another.
'book' becomes 'nt' (New Testament).
'tried' is an anagram of 'tride'.
'tride'+'nt'='TRIDENT'
(Other definitions for trident that I've seen before include "Fork perhaps" , "Weapon on submarine" , "Neptune's spear - missile" , "Spear with three prongs" , "3-pronged spear" .)