Clement Acknowledges He Called the Collection of Pauline Letters 'the Apostolikon'

I really don't know the answer and I suspect that none of the 'experts' do either.  It's a very difficult question to answer given the lack of evidence we have available to us.  Yes it means 'something like' evangelic and apostolic.  But that isn't really helpful in and of itself.  We know the Marcionites thought that the same person wrote the 'gospel' and 'apostolic' letters so it would seem to indicate some characteristic of the collection rather than an association with two separate individuals - i.e. an 'evangelist' (Mark) and an 'apostle' (Paul).

All that is clear to me at least is that Clement of Alexandria makes reference to the very same Marcionite division of the New Testament into 'the Evangelikon' and 'the Apostolikon' which is his student Origen - now under the sway of Irenaeus's teaching - criticizes in his other works.  We read Clement conclude the Stromata with the reference to a particular part of the Apostolikon noting:

For in the first Epistle to the Corinthians the divine apostle (ὁ θεῖος ἀπόστολος) says: "Dare any of you, having a matter against the other, go to law before the unrighteous, and not before the saints? Know ye not that the saints shall judge the world?" and so on. The section being very long, we shall exhibit the meaning of the apostle's utterance (τοῦ ἀποστόλου παραστήσομεν) by employing such of the expressions of the Apostolikon (τῶν ἀποστολικῶν) as are most pertinent, and in the briefest language, and in a sort of cursory way, interpreting the discourse in which he describes the perfection of the Gnostic. [Strom 7.14]

I think this once again reinforces (a) that Clement was a crypto-Marcionite and that (b) Origen seems to have moved away from his teachings, perhaps only hypocritically.