Answer: auxiliary

AnswerCrossword Clue
AUXILIARYReserve
auxiliaryProviding supplementary or additional help and support
auxiliarya helper, a subordinate also AUXILIAR
auxiliary(of troops) Engaged in the service of a nation at war but not part of the regular army, and often of foreign origin
auxiliary(of equipment) Held in reserve
auxiliary(of a sailing vessel) Equipped with a supplementary engine