Box traitor inside church (5)
I believe the answer is:
crate
'box' is the definition.
(I know that crate is a type of box)
'traitor inside church' is the wordplay.
'traitor' becomes 'rat' (rat can mean a traitor).
'inside' indicates putting letters inside.
'church' becomes 'CE' (abbreviation for Church of England).
'rat' inserted inside 'ce' is 'CRATE'.
(Other definitions for crate that I've seen before include "Packing-case made of wooden slats" , "Jalopy" , "Wooden container" , "Packing case with wooden slats" , "Clapped-out vehicle" .)