Turn on charm, mostly to get the honest truth (6)
I believe the answer is:
gospel
'the honest truth' is the definition.
(thesaurus)
'turn on charm mostly' is the wordplay.
'turn' becomes 'go' (a person's move in a board game).
'on' says to put letters next to each other.
'charm' becomes 'spell' (both can mean a bit of magic).
'mostly' means to remove the last letter.
'spell' with its final letter removed is 'spel'.
'go'+'spel'='GOSPEL'
'to get' is the link.
(Other definitions for gospel that I've seen before include "Absolute truth" , "One of the first four Books of the New Testament" , "The teaching of Christ" , "Evangel" , "Christian doctrine" .)