a minor actor in crowd scenes同义词：supernumerary, spear carrier
an additional edition of a newspaper (usually to report a crisis)
something additional of the same kind同义词：duplicate
形容词 extra:
more than is needed, desired, or required同义词：excess, redundant, spare, supererogatory, superfluous, supernumerary, surplus
added to a regular schedule同义词：special
further or added同义词：additional
副词 extra:
unusually or exceptionally
例句
Luggage in excess of 100 kg will be charged extra.超过100公斤的行李要额外收费。
I'm so envious of you getting an extra day's holiday.我真羡慕你得到一天额外的假期。
The teacher disciplined the class by giving them extra homework.那教师布置额外的家庭作业来处罚学生。
He paid the extra cost without a murmur.他付了额外的费用而毫无怨言。
This dress is of extra fine quality.这件衣服的质地特别好。
There were so many people that the company put on extra buses.人数太多，公司加开了公共汽车。
详细解释
ex.tra adj.Abbr. ext.,ex.,X（形容词）缩写 ext.,ex.,XMore than or beyond what is usual, normal, expected, or necessary.See Synonyms at superfluous 超额的，额外的：多于或超过通常的、正常的、意料的或必要的参见 superfluousBetter than ordinary; superior:特佳的：比正常的好；优秀的：例句：extra fineness.特别好 Subject to an additional charge:另收费的：须经额外加费的：例句：a pizza with extra cheese.奶酪要另行收费的比萨饼 n.（名词）Something more than is usual or necessary.额外物：超出平常的或必要的东西Something, such as an accessory on a motor vehicle, for which an additional charge is made.附加物：须另外收费的某物，如机动车上的附件A special edition of a newspaper.号外：报纸中特殊的一版 An additional or alternate worker.额外的人手A performer hired to play a minor part, as in a crowd scene.临时演员：如拍摄电影群众场面时雇来的临时演员Something of exceptional quality.上品，特级品：质量特别好的东西adv.（副词）To an exceptional extent or degree; unusually:格外地，特别地：达到例外的程度或限度；格外地：例句：extra dry.格外干 来源：Probably short for extraordinary 可能为 extraordinary的简写