That quote is seriously mangled. Old English spelling was fluid and not particularly standardised, but this quote is just wrong.
The quote is from King Alfred’s preface to Pope Gregory’s Pastoral Care, and it should read (in Robert E. Diamond’s version linked to):
Swǣ clǣne hēo wæs oþ-feallenu on Angelcynne þæt swīðe fēawa wǣron be-heonan Humbre þe hiora þeȝnunga cūðen onder-standan on Englisc …
The translation is reasonably sound, though perhaps not too easy to understand.
Fallen away (oþ-feallenu) here means ‘decayed, fallen in standard’, and clean (clǣne) is an adverb meaning ‘entirely, completely’, similar to how it’s used in Modern English in contexts like, “I clean forgot about that”. Service books (þeȝnunga) are books used for service/mass in church.
The ‘it’ being talked about is, it seems, learning. Alfred is talking about how sad it is that England, once a stronghold of learning and wisdom, has now (well, back then) become a country of imbeciles and ignoramuses which would have to import its wisdom from elsewhere. In context, Diamond’s far superior translation reads:
[It has very often come to my mind] how abroad one looked to this country for learning and instruction, and how we should now get it from abroad, if we were to have it. So completely had it lapsed in England that there were very few on this side of the Humber who could understand their mass-books in English or translate even one written message from Latin into English …