part of speech - "微" 管仲，吾其被髮左衽矣。
微 should be an adverb here. The sentence means "如果没有管仲的话, ..." (if there was no 管仲, then ...), and 微 is not an action here.


  微 wēi
  
  〈副>
  
  要没有,要不是 [but for;if it were not for]
  
  微斯人,吾谁与归。——宋· 范仲淹《岳阳楼记》