Have you tried setting the files in the same way as the docs? This specifies the source files slightly differently to how you have it:
less: {
development: {
options: {
paths: ["assets/css"]
},
files: {
"path/to/result.css": "path/to/source.less"
}
},
production: {
options: {
paths: ["assets/css"],
cleancss: true,
modifyVars: {
imgPath: '"http://mycdn.com/path/to/images"',
bgColor: 'red'
}
},
files: {
"path/to/result.css": "path/to/source.less"
}
}
}
See the usage examples in the docs for more clarification: https://github.com/gruntjs/grunt-contrib-less#usage-examples