Mars passes round mid-air (5)
I believe the answer is:
ruins
'mars' is the definition.
(I've seen this before)
'passes round mid-air' is the wordplay.
'passes' becomes 'runs' (running is a kind of passing**).
'round' is an insertion indicator.
'mid' indicates the central letters.
The central letter of 'air' is 'i'.
'runs' enclosing 'i' is 'RUINS'.
(Other definitions for ruins that I've seen before include "Derelict buildings" , "Collapsed, disintegrated buildings" , "Wrecked buildings" , "Spoils" , "Mars" .)