How to use clef "moderntab"

[wjm]

Greetings,
You wrote
++++++++++++++++++++++++++++++
\new StaffGroup <<
    \new Staff \symbols {
    \clef "bass_8"
    }
    \new TabStaff \symbols {
    \clef moderntab
    \set TabStaff.stringTunings = #bass-tuning
    }
>>
++++++++++++++++++++++++++++++
From the Notation Reference - slightly edited

\new TabStaff
{
  \clef moderntab
  \music
}

Note the ordering of the 'words' ie the syntax.
:)
Hope this helps

Cheers
Bill