One means of improving athletic performance: live with military officer somewhere beyond Mars! (8,4)
I believe the answer is:
asteroid belt
'somewhere beyond mars' is the definition.
(area of the solar system)
'one means of improving athletic performance live with military officer' is the wordplay.
'one' becomes 'a' (a thing is one thing).
'means of improving athletic performance' becomes 'steroid' (steroids might improve athletic performance).
'live' becomes 'be' (synonyms).
'with' is a charade indicator (letters next to each other).
'military officer' becomes 'lt' (lieutenant).
'a'+'steroid'+'be'+'lt'='ASTEROID BELT'
(Other definitions for asteroid belt that I've seen before include "Region of interplanetary space - battered soil (anag)" , "Region of space between Mars and Jupiter - besotted liar (anag)" , "Zone where many minor planets orbit" .)