Effect  of A-site disorder in half-doped La_0.5Ca_0.5MnO₃ created by the substitution of Tb at La site is studied through temperature dependent neutron diffraction, resistivity, and magnetization on La_0.375Tb_0.125Ca_0.5MnO₃ (LTC) to identify the evolution of phases as a function of temperature and magnetic field. Contrary to the parent system, the paramagnetic(PM)to ferromagnetic(FM) transition is found to be of first order but becomes observable only in fields above 1T. Absence of this transition in zero and low fields along with the observation of a very intriguing and irreversible  M–H curve at  low temperature raises the question about the ground state of the system. It is identified from the specially designed measurement protocol that the ground state of the system becomes FM, contrary to the parent LCMO. However, this remains masked because of the hindrance to the transformation kinetics of the PM–FM first or dertransition in low fields. Significantly, though the thermal hysteresis of the first- order transition decreases with the measurement field, collapse of such a behavior to a critical point is not observed in LTC, unlike earlier observations in some other manganites.