Mars journeys must include one (5)
I believe the answer is:
ruins
'mars' is the definition.
(I've seen this before)
'journeys must include one' is the wordplay.
'journeys' becomes 'runs' (run is a kind of journey).
'must include' is an insertion indicator.
'one' becomes 'i' (Roman numeral).
'runs' enclosing 'i' is 'RUINS'.
(Other definitions for ruins that I've seen before include "Damages irreparably" , "Spoils" , "Destroys" , "Collapsed, disintegrated buildings" , "Derelict remains of building" .)