Two turns, but one left out the truth (6)
I believe the answer is:
gospel
'truth' is the definition.
(e.g. you can take is as gospel)
'two turns but one left out' is the wordplay.
'two turns' means two substitutions for 'turn'.
'turn' becomes 'go' (a person's move in a board game).
'turn' says the letters should be written in reverse.
'but one left out' becomes 'leps' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'leps' reversed gives 'spel'.
'go'+'spel'='GOSPEL'
'the' is the link.
Can you help me to learn more?
(Other definitions for gospel that I've seen before include "Mark, perhaps" , "Type of music" , "One of the first four Books of the New Testament" , "The teachings of Christ in a Book of the New Testament" , "Holy book" .)