Moving version of Tristan (7)
I believe the answer is:
transit
'moving' is the definition.
(I've seen this before)
'version of tristan' is the wordplay.
'version of' is an anagram indicator (another version of the same letters).
'tristan' is an anagram of 'TRANSIT'.
(Other definitions for transit that I've seen before include "Shipment from A to B" , "The process of going or conveying" , "Passage of planet across the disc of the sun" , "In migration" , "To pass through/across" .)