Converting 3x to 1x Drivetrain
General consensus is that a 'clutched' rear derailleur is also a key component to a good/successful 1x setup. These derailleurs provide a tighter 'pull' on the chain and help keep it from hopping off the chainring on rough terrain.

For cassette/freehub compatibility, read the discussion that occurred here: 
https://bicycles.stackexchange.com/a/28908/30533 ...


  Shimano/SRAM 11 speed cassettes are wider than 8/9/10 speed ones. So yes, you need a new, wider freehub body, unless your old one was not very old and used a spacer to fit a 10-speed cassette.