Leave talk, missing one truthful statement (6)
I believe the answer is:
gospel
'one truthful statement' is the definition.
(gospel can mean something assumed to be true)
'leave talk missing' is the wordplay.
I cannot really understand how this works, but
'leave' could be 'go' (to go is to depart or leave) and 'go' is found in the answer.
This may be the basis of the clue (or it may be nonsense).
(Other definitions for gospel that I've seen before include "The teaching of Christ" , "Mark, perhaps" , "Holy book" , "One in quartet" , "Truth (religious)" .)