We exploit the gravitational potential of a massive , rich cluster at z = 0.9 to study the internal properties of a gravitationally lensed galaxy at z =4.88 . Using high resolution HST imaging together with optical ( VIMOS ) and near-infrared ( SINFONI ) Integral Field Spectroscopy we have studied the rest-frame UV and optical properties of the lensed galaxy seen through the cluster RCS0224-002 . Using a detailed gravitational lens model of the cluster we reconstruct the source-frame morphology on 200pc scales and find an \sim L ^ { * } Lyman-break galaxy with an intrinsic size of only 2.0 \times 0.8 kpc , a velocity gradient of \mathrel { \raise 1.505 pt \hbox { $ \scriptstyle < $ } \kern - 6.0 pt \lower 1.72 pt \hbox { { $% \scriptstyle \sim$ } } } 60 \hbox { km } \hbox { s } ^ { -1 } and an implied dynamical mass of 1.0 \times 10 ^ { 10 } \mathrel { M _ { \odot } } within 2 kpc . We infer an integrated star-formation rate of just 12 \pm 2 \mathrel { M _ { \odot } \hbox { yr } ^ { -1 } } from the intrinsic [ O ii ] \lambda 3727 emission line flux . The Ly \alpha emission appears redshifted by +200 \pm 40 \hbox { km } \hbox { s } ^ { -1 } with respect to the [ O ii ] emission . The Ly \alpha is also significantly more extended than the nebular emission , extending over 11.9 \times 2.4 kpc . Over this area , the Ly \alpha centroid varies by less than 10 \hbox { km } \hbox { s } ^ { -1 } . We model the asymmetric Ly \alpha emission with an underlying Gaussian profile with an absorber in the blue wing and find that the underlying Ly \alpha emission line centroid is in excellent agreement with the [ O ii ] emission line redshift . By examining the spatially resolved structure of the [ O ii ] and Ly \alpha emission lines we investigate the nature of this system . The model for local starburst galaxies suggested by Mass-Hesse et al . ( 2003 ) provides a good description of our data , and suggests that the galaxy is surrounded by a galactic-scale bi-polar outflow which has recently burst out of the system . The outflow , which appears to be currently located \mathrel { \raise 1.505 pt \hbox { $ \scriptstyle > $ } \kern - 6.0 pt \lower 1.72 pt \hbox { { $% \scriptstyle \sim$ } } } 30 kpc from the galaxy , is escaping at a speed of upto \sim 500 \hbox { km } \hbox { s } ^ { -1 } . Although the mass of the outflow is uncertain , the geometry and velocity of the outflow suggests that the ejected material is travelling far faster than escape velocity and will travel more than 1 Mpc ( comoving ) before eventually stalling .