I am using grunt-contrib-less for compiling all less files. I found two options doing the same for me.
You can't use both, as the docs tell you
compress Compress output by removing some whitespaces. If you would like more involved minification you should use the Clean CSS option.
compress option are built in the Less compiler and
clean-css are separate package.
Currently the Clean CSS option also strips (or does not insert) the
/*# sourceMappingURL= comment., see: https://github.com/less/less.js/issues/1656
The Less compiler also has the possibility to pass an option to clean css with
clean-option grunt-contrib-less does not seems to support this option.