The full phrase (v2) almost certainly means "prominent among the apostles," rather than "outstanding among the apostles" (see, e.g. RSV, NEB, NIV, NJB, Cranfield, Kasemann and those cited by him). The straightforward description "the apostles" (contrast 2 Cor 8:23 and Phil 2:25), and the following clause, together strongly suggest that Andronicus and Junia belonged to the large group (larger than the twelve) of those appointed apostles by the risen Christ in 1 Cor 15:7 (Schnackenburg, "Apostles," 293-94, surprisingly ignores 1 Cor 15:7 when he argues that Andronicus and Junia [sic] had never "seen the risen Lord"). That is, they belonged most probably to the closed group following apostles appointed directly by the risen Christ in a limited period following his resurrection (Paul himself being aborted = born early, in order to enter this circle is uniquely appointed apostles before it closed--"last of all as to the abortion" [1 Cor 15:8]). This would give Andronicus and Junia a higher status in the eyes of Paul and others. . .We may firmly conclude, however, that one of the fiondation apostles of Christiniaty was a woman and wife."
James D.G. Dunn, "Romans 9-16" pg 894-95.I'm not certain Junia was the wife of Andronicus, as it is never explicitly stated in the text. the inference could be there, but as it simply isn't explicitly stated, I'm content to step back and shrug. I'm with Dunn on this one.