You can use the sortname field to specify exactly under which name (and where) the entry ought to be sorted (see p. 27 of the biblatex documentation).
What you have to keep in mind here is that Biber's sorting algorithm does apply the nosort options to the author and editor field and does remove the "al-" there prior to sorting, but does no such thing to the sortname field. (Whether or not that is by intention or should be considered a bug is up for discussion. I for one thought this behaviour quite unusual at first [technically Biber does not consider sortname to be of type_name], but I can definitely see the benefit of not touching the sortname field in any way and actually think this is the "correct" behaviour.)
Refer to §3.1.6. The nosort option, p. 27, of the Biber documentation for more on the nosort option, its default value and what you can do with it.
So in sortname you need to give the name as you would like to have it sorted.
author = {\autocap{a}z-Zayn, Ahmad}
sortname = {Zayn, Ahmad}
or
author = {\autocap{a}l-Kabīr}
sortname = {Kabīr}
sortnameis for (see p. 27 of thebiblatexdoc). You would probably haveauthor = {\autocap{a}l-Kabīr}for right capitalisation andsortname = {al-Kabīr},for correct sorting. – moewe Aug 02 '15 at 15:30labelalpha=trueand changing tosorting=anytas per the discussion in http://tex.stackexchange.com/questions/42382/sorting-with-biblatex-using-sortname. Still doesn't work, though. Could there be a bug in Biblatex (that's what's suggested over at the other question). – Sixtyfive Aug 02 '15 at 16:28labelalpha=trueandsorting=anytexcept of course if you want to use analphabeticstyle instead of anauthoryearone. – moewe Aug 02 '15 at 16:35biblatex, but maybe I'm not getting what he hinted at. – moewe Aug 02 '15 at 16:40sortnamedoes not get the same stripping of diacritics and prefix treatment asauthor/editorand is read largely as is, so you need to get rid of theal-prefix yourself and cannot rely on Biber to do so.sortname = {Kabīr}should work. – moewe Aug 02 '15 at 16:46type_nameare "strip[ped of] prefices like 'al-'", in the table on p. 28 you can see thatsortnameis not one of the fields oftype_nameand thus does not get that treatment. (The fieldsauthorandeditordo get this treatment, this is whyauthor = {al-Kabīr}sorts under "K".) – moewe Aug 02 '15 at 17:07sortnamefield. It works just fine now, of course, withoutlabelalpha=trueand withsorting=nyt. Is there any way to get your comment converted to an answer so that I can accept it? – Sixtyfive Aug 02 '15 at 18:39