Travellers in train set out (10)
'travellers' is the definition.
(itinerant is a kind of traveller)
'in train set out' is the wordplay.
'out' indicates anagramming the letters (out can mean wrong or inaccurate).
'intrainset' anagrammed gives 'ITINERANTS'.
(Other definitions for itinerants that I've seen before include "Point and talk wildly in view of island", "Travellers".)