We present a complete next-to-leading order QCD calculation for
single-inclusive large-pT hadron production in longitudinally polarized
lepton-nucleon collisions, consistently including ``direct'' and ``resolved''
photon contributions. This process could be studied experimentally at a future
polarized lepton-proton collider like eRHIC at BNL. We examine the sensitivity
of such measurements to the so far completely unknown parton content of
circularly polarized photons.Comment: 15 pages, 7 eps figure