One true turning on the way (2,5)
I believe the answer is:
en route
'on the way' is the definition.
(synonyms)
'one true turning' is the wordplay.
'turning' is an anagram indicator.
'one'+'true'='onetrue'
'onetrue' is an anagram of 'EN ROUTE'.
(Other definitions for en route that I've seen before include "As part of a journey" , "While travelling" , "On the way - in France" , "March" , "In transit (French)" .)