  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 ~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_* = 2 x 10^9
solar masses) with a high specific star formation rate (~20 /Gyr). From the
blue emission lines we measure a gas-phase oxygen abundance consistent with
solar (12+log(O/H)=8.8 +/- 0.2). We break the continuous line-emitting region
of this giant arc into seven ~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 ~1kpc have a placement on the
blue HII region excitation diagram with f([OIII])/f(Hbeta) and
f([NeIII])/f(Hbeta) 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.
