One of the intriguing phenomena in these materials is superconductivity which was observed in NbSe_2, gated MoS_2 and others. The precise physical picture of superconducting state is still under debate in the scientific community. The aim of this letter is to shed the light on some collective properties of such a system and how they are related to the pairing between electrons. The effective low-energy action for fluctuations of the phase of the order parameter was derived, and collective subgap excitations in the system were analyzed. It was shown that for nearly equal singlet and triplet coupling constants, there is a special collective mode of Leggett type, which is gapped and becomes softer if parallel magnetic field to the system is applied. These results open the possibility to identify this collective mode on experiment and shed the light on the relevant pairing mechanism.