Cluster star-forming galaxies are found to have an excess of far-infrared emission relative to H{alpha}, when compared to those in the field, which could be caused by intense active galactic nuclei (AGN) activity, dust and/or declining star formation histories. Here we present spectroscopic observations of H{alpha} emitters in the Cl 0939+4713 (Abell 851) super-cluster at z=0.41, using AF2+ WYFFOS on the William Herschel Telescope. We measure [OII], H{beta}, [OIII], H{alpha} and [NII] for a sample of 119 H{alpha} emitters in and around the cluster. We find that 17+/-5 per cent of the H{alpha} emitters are AGN, irrespective of environment. For star-forming galaxies, we obtain Balmer decrements, metallicities and ionization parameters with different methods, individually and by stacking. We find a strong mass-metallicity relation at all environments, with no significant dependence on environment. The ionization parameter declines with increasing stellar mass for low-mass galaxies. H{alpha} emitters residing in intermediate environments show the highest ionization parameters (along with high [OIII]/H{alpha} and high [OIII]/[OII] line ratios, typically twice as large as in the highest and lowest densities), which decline with increasing environmental density. Dust extinction (A_H{alpha}_) correlates strongly with stellar mass, but also with environmental density. Star-forming galaxies in the densest environments are found to be significantly dustier (A_H{alpha}_~=1.5-1.6) than those residing in the lowest density environments (A_H{alpha}_~=0.6), deviating significantly from what would be predicted given their stellar masses.