Military tenure (4)
I believe the answer is:
tour
'military tenure' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for tour that I've seen before include "Period of duty" , "Rout on excursion" , "... de France is a multi-stage cycle race" , "A sightseeing trip or journey" , "Trip visiting several places" .)