\version "2.11.41"
myclef = {
  \set Staff.clefGlyph = #"clefs.neomensural.c"
  \set Staff.clefPosition = #1
  \set Staff.middleCPosition = #1 
}

#(set! supported-clefs (cons '("neomensural-cc" . ("clefs.neomensural.c" 1 0)) supported-clefs))

<<
  \context Staff = "Manual" <<
    \myclef
    \relative c' { <c e>8 <c e>8 <c f>8 <c f>8 }
  >>
  \context Staff = "ClefDef"<<
      \clef "neomensural-cc"
      \relative c' { <c e>8 <c e>8 <c f>8 <c f>8 }
    >>
>>