Import HEVC video from iPhone 7 to Final Cut Pro
The only solution until Apple release the next version of Final Cut Pro X is to not shoot in HEVC, or convert before moving to FCP.


  If you haven’t yet shot video you need to edit with FCP, there’s an
  easy fix.
  
  
  Go into the phone’s Settings and choose Camera. Then choose Formats.
  There you’ll find two options. By default, High Efficiency will be
  checked. You want to check Most Compatible.
  
  
  If you have existing HEVC footage you need to edit in FCP, there’s a
  slightly less easy workaround.
  
  
  Import the clip into the Photos app on your Mac. Then, once it’s fully
  imported (which could take a little while depending on how much video
  we’re talking about here), then just select the clip(s) you need and
  Export them from Photos. Photos will export them with a codec that
  Final Cut Pro X can read. So once the clip(s) is/are exported from
  Photos to whatever location you chose, then open Final Cut Pro and
  import the clip(s) you just exported from Photos.  
  


Source: You can’t yet edit iOS 11 iPhone video in FCP X, fix likely coming soon