At certain conditions, a fast instability is observed in Recycler. It develops in tens of revolutions and may lead to increase of beam emittance and beam loss. Various peculiar features of the instability: its occurrence only above a certain intensity threshold, and only in horizontal plane, as well as the rate of the instability suggest that its nature is electron cloud. Observations of beam dynamics, in particular the stabilization of the beam motion by a low intensity clearing bunch, confirm the electron cloud nature of the instability. The findings suggest the trapping of the electron cloud in Recycler combined function magnets. According to numerical simulations in the PEI code, in a combined function magnet the electron cloud is located at the beam center and up to 1% of the particles can be trapped by the magnetic field. Since the process of electron cloud build-up is exponential, once trapped this amount of electrons significantly increases the density of the cloud on the next revolution. In a Recycler combined function dipole this multi-turn accumulation allows the electron cloud reaching final intensities orders of magnitude greater than in a pure dipole. A simple analytical model has been developed to estimate the parameters of the multibunch cloud driven instability with the electrons trapped in combined function dipoles. The resulting instability growth rate of about 30 revolutions and the mode frequency less than 1 MHz are consistent with experimental observations and qualitatively agree with the numerical simulations. The model allows an estimation of the instability rate for the future intensity upgrades.