Once this was the standard way of forming the past participle of the verb clepe, to call (or, more strictly, its Old English precursor, cleopian).
For the past few hundred years it has only turned up as a deliberately archaic form, mostly in poetry, or as light relief. It surfaces occasionally as ponderous humour in journalistic pieces, as here in the Jerusalem Post in 1997 (the name of the writer has been suppressed to protect the guilty): “The caption under the photo of the unfortunately yclept basketball player just makes matters worse: ‘David Putz dribbles away...’ ”.
Such poppings-up are frequent enough that the word appears in most dictionaries today, even though it died out in the north of England about 1200 and lingered in the south and east only a little longer. It has been outside the mainstream of English for so long that the person credited with popularising it again (Gavin Douglas, a Scots poet and divine), wrote around the end of the fifteenth century.
The initial y was once the standard way of marking the past participle: yclensed, yfastened, ypunched, and dozens of others. It was the Old English equivalent of a form which still exists, for example, as ge– in modern German. Advice to budding writers: best avoided!