Clue: Big beast's tail bitten off by antelope in demilitarised zone once (9)
Answer: RHINELAND
Explanation: Definition: demilitarised zone once
A charade of RHIN[o] ('big beast') minus its last letter ('tail bitten off') plus ELAND ('antelope'). The RHINELAND was off limits to the German military after WWI, until German forces re-entered in 1936.