1. 清除;清理
    Clearance is the removal of old buildings, trees, or other things that are not wanted from an area.

    e.g. ...a slum clearance operation in Nairobi...
           内罗毕拆除贫民窟的行动
    e.g. The UN pledged to help supervise the clearance of mines.
           联合国承诺协助监督扫雷工作。

2. (官方的)批准，许可
    If you get clearance to do or have something, you get official approval or permission to do or have it.

clearance的反义词

    e.g. Thai Airways said the plane had been given clearance to land...
           泰国航空公司说，该飞机已经得到降落许可。
    e.g. He has a security clearance that allows him access to classified information.
           他持有安全许可，可以接触机密信息。

3. (路面、河面与上方桥梁之间的)净空，间隙，间距
    The clearance of a bridge is the distance between the lowest point of the bridge and the road or the water under the bridge.

    e.g. The lowest fixed bridge has 12.8m clearance.
           最低的固定桥下方通行高度为12.8米。