[en] Mass imbalance in strongly interacting mixtures of ultracold fermions is predicted to lead to new pairing phenomena and quantum phases. We investigate a mass-imbalanced ⁶Li-⁴⁰K Fermi-Fermi mixture in the regime of strong interactions on the repulsive side of an interspecies Feshbach resonance. We find that, for a sufficiently strong repulsive s-wave interaction, the ⁴⁰K atoms and the ⁶Li⁴⁰K dimers interact attractively, which is in strong contrast to the mass-balanced case. This surprising behavior is related to the existence of a ↑↑↓ trimer state in ↑↓ Fermi-Fermi mixtures with a mass ratio m_↑/m_↓ > 8.2. For lower mass ratios (i.e. m_K/m_Li = 6.64) this trimer state turns into a p-wave atom-dimer scattering resonance. Here, we present our experimental results on interactions in a resonantly interacting atom-dimer mixture. Employing radio-frequency spectroscopy over a range of temperatures and interaction strengths, we confirm the presence of a strong attraction on the repulsive side of a Feshbach resonance, in good agreement with theory.