The entanglement of charge density wave (CDW), superconductivity, and topologically nontrivial electronic structure has recently been discovered in the kagome metal $A$V$_3$Sb$_5$ ($A$ = K, Rb, Cs) family. With high-resolution angle-resolved photoemission spectroscopy, we study the electronic properties of CsV$_3$Sb$_5$ deep in the CDW state. The spectra around $bar{K}$ is found to exhibit a peak-dip-hump structure associated with two separate branches of dispersion, demonstrating the isotropic CDW gap opening. The peak-dip-hump lineshape is contributed by linearly dispersive Dirac bands in the lower branch and a dispersionless flat band close to $E_{rm F}$ in the upper branch. The Fermi surface nesting scenario can account for these CDW-related features. The high density of states at $E_{rm F}$ associated with the flat band could play an essential role in the onset of superconductivity.
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