Set the following in your init.vim
. The backslash is required.
let g:clipboard = #{
\ name: 'xsel',
\ copy: {
\ '+': ['xsel', '--nodetach', '-i', '-b'],
\ '*': ['xsel', '--nodetach', '-i', '-p'],
\ },
\ paste: {
\ '+': ['xsel', '-o', '-b'],
\ '*': ['xsel', '-o', '-p'],
\ },
\ cache_enabled: 1,
\ }