We present a Very Large Telescope/Multi Unit Spectroscopic Explorer (MUSE) survey of lensed high-redshift galaxies behind the z = 0.77 cluster RCS 0224-0002. We study the detailed internal properties of a highly magnified (mu similar to 29) z = 4.88 galaxy seen through the cluster. We detect widespread nebular C IV lambda lambda 1548,1551 angstrom emission from this galaxy as well as a bright Ly alpha halo with a spatially uniform wind and absorption profile across 12 kpc in the image plane. Blueshifted high-and low-ionization interstellar absorption indicate the presence of a high-velocity outflow (Delta v similar to 300 km s(-1)) from the galaxy. Unlike similar observations of galaxies at z similar to 2 - 3, the Lya emission from the halo emerges close to the systemic velocity - an order of magnitude lower in velocity offset than predicted in 'shell'-like outflow models. To explain these observations, we favour a model of an outflow with a strong velocity gradient, which changes the effective column density seen by the Lya photons. We also search for high-redshift Ly alpha emitters and identify 14 candidates between z = 4.8 - 6.6, including an overdensity at z = 4.88, of which only one has a detected counterpart in Hubble Space Telescope/Advanced Camera for Surveys+Wide Field Camera 3 imaging.