Red supergiant stars are surrounded by a gaseous and dusty circumstellar environment created by their mass loss which spreads heavy elements into the interstellar medium. The structure and the dynamics of this envelope are crucial to understand the processes driving the red supergiant mass loss and the shaping of the pre-supernova ejecta. We have observed the emission from the CO $J = 2-1$ line from the red supergiant star $mu$~Cep with the NOEMA interferometer. In the line the synthesized beam was $0.92 times 0.72$~arcsec ($590 times 462$~au at 641~pc). The continuum map shows only the unresolved contribution of the free-free emission of the star chromosphere. The continuum-subtracted channel maps reveal a very inhomogeneous and clumpy circumstellar environment. In particular, we detected a bright CO clump, as bright as the central source in the line, at 1.80~arcsec south-west from the star, in the blue channel maps. After a deprojection of the radial velocity assuming two different constant wind velocities, the observations were modelled using the 3D radiative transfer code textsc{lime} to derive the characteristics of the different structures. We determine that the gaseous clumps observed around $mu$~Cep are responsible for a mass loss rate of $(4.9 pm 1.0) times 10^{-7}~{rm M}_odot,{rm yr}^{-1}$, in addition to a spatially unresolved wind component with an estimated mass-loss rate of $2.0 times 10^{-6}~{rm M}_odot,{rm yr}^{-1}$. Therefore, the clumps have a significant role in $mu$~Ceps mass loss ($ge 25 %$). We cannot exclude that the unresolved central outflow may be made of smaller unresolved clumps.
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