  In certain unconventional superconductors with sizable electronic
correlations, the availability of closely competing pairing channels leads to
characteristic soft collective fluctuations of the order parameters, which
leave fingerprints in many observables and allow to scrutinize the phase
competition. Superconducting layered materials, where electron-electron
interactions are enhanced with decreasing thickness, are promising candidates
to display these correlation effects. In this work, we report the existence of
a soft collective mode in single-layer NbSe$_2$, observed as a characteristic
resonance excitation in high resolution tunneling spectra. This resonance is
observed along with higher harmonics, its frequency $\Omega/2\Delta$ is
anticorrelated with the local superconducting gap $\Delta$, and its amplitude
gradually vanishes by increasing the temperature and upon applying a magnetic
field up to the critical values (T$_C$ and H$_{C2}$), which sets an unambiguous
link to the superconducting state. Aided by a microscopic model that captures
the main experimental observations, we interpret this resonance as a collective
Leggett mode that represents the fluctuation towards a proximate f-wave triplet
state, due to subleading attraction in the triplet channel. Our findings
demonstrate the fundamental role of correlations in superconducting 2D
transition metal dichalcogenides, opening a path towards unconventional
superconductivity in simple, scalable and transferable 2D superconductors.
