If it helps with troubleshooting, I've also tried to do this with \fastsetup instead of \texdefinition:

```
\expanded{\abbreviation[\currentbtxtag]{\fastsetup{btx:sbl:cite:shorthand}}{\fastsetup{btx:sbl:cite:listsubcite}}}
```

This also doesn't work, but I at least get a "Missing control sequence inserted" error. The following MWE should reproduce the error (it modifies the existing Chicago rendering, but since I couldn't figure out how to add a shorthand field to the @book category in the chicago spec from within the MWE, I just used the note field as a placeholder for it):

```

% Setup a minimal bibliography
\startbuffer [bib]
@book{Author1:2016,
author={Author1},
title={Work1},
note={W1},
publisher={Publisher1},
year=2016,
}
@book{Author2:2015,
author={Author2},
title={Work2},
publisher={Publisher2},
year=2015,
}
\stopbuffer
\usebtxdataset [bib.buffer]

% Setup a new cite alternative to test the desired functionality
\startbtxrenderingdefinitions[chicago]
\definesynonyms[abbreviation][abbreviations][\infull][\inshort] % we need the \inshort command to access abbreviations by their entry tag

\startsetups btx:chicago:cite:shorthand
\fastsetup{btx:cite:concat}
\fastsetup{btx:cite:lefttext}
\begingroup
\def\currentbtxcitealternative{short}
\def\currentbtxcategory{\btxfield{category}}
\btxstartstyleandcolor[sbl:\s!list:title:\currentbtxcategory]
            \btxusecommand[sbl:\s!list:title:\currentbtxcategory]{
              \btxflush{note}% set the shorthand according to the current category
            }
          \btxstopstyleandcolor
\endgroup
\fastsetup{btx:cite:righttext}
\stopsetups

\startsetups btx:chicago:cite:test
\fastsetup{btx:cite:concat}
\fastsetup{btx:cite:lefttext}
\btxdoifelse{note} {
 %\expanded{\definesynonym[yes][abbreviation][\currentbtxtag]{\fastsetup{btx:chicago:cite:shorthand}}{\fastsetup{btx:cite:entry}}}% this throws a Missing control sequence inserted error
 \expanded{\definesynonym[yes][abbreviation][\currentbtxtag]{\cite[alternative=shorthand][\currentbtxtag]}{\cite[alternative=entry][\currentbtxtag]}}% this works, but recursively cites the same tag
 \inshort{\currentbtxtag}
} {
 \fastsetup{btx:cite:entry}
}
\fastsetup{btx:cite:righttext}
\stopsetups
\stopbtxrenderingdefinitions

% Use this cite alternative by default
\usebtxdefinitions[chicago]
\setupbtx[chicago:cite][alternative=test]

\starttext
\cite[Author1:2016]\blank
\cite[Author2:2015]\blank
\page
\startsubject[title=Abbreviations]
\placelistofabbreviations
\stopsubject
\page
\startsubject[title=Bibliography]
\placelistofpublications
\stopsubject
\stoptext

```

Joey