We take advantage of gravitational lensing amplification by Abell 1689 ( z =0.187 ) to undertake the first space-based census of emission line galaxies ( ELGs ) in the field of a massive lensing cluster . Forty-three ELGs are identified to a flux of i _ { 775 } =27.3 via slitless grism spectroscopy . One ELG ( at z =0.7895 ) is very bright owing to lensing magnification by a factor of \approx 4.5 . Several Balmer emission lines detected from ground-based follow-up spectroscopy signal the onset of a major starburst for this low-mass galaxy ( M _ { * } \approx 2 \times 10 ^ { 9 } M _ { \odot } ) with a high specific star formation rate ( \approx 20 Gyr ^ { -1 } ) . From the blue emission lines we measure a gas-phase oxygen abundance consistent with solar ( 12+log ( O/H ) =8.8 \pm 0.2 ) . We break the continuous line-emitting region of this giant arc into seven \sim 1kpc bins ( intrinsic size ) and measure a variety of metallicity dependent line ratios . A weak trend of increasing metal fraction is seen toward the dynamical center of the galaxy . Interestingly , the metal line ratios in a region offset from the center by \sim 1kpc have a placement on the blue HII region excitation diagram with f ( [ OIII ] ) / f ( H \beta ) and f ( [ NeIII ] ) / f ( H \beta ) that can be fit by an AGN . This asymmetrical AGN-like behavior is interpreted as a product of shocks in the direction of the galaxy ’ s extended tail , possibly instigated by a recent galaxy interaction .