\definemathfraction
  [wfrac]
  [margin=.25em]

\starttext

\startformula
    \wfrac {
        a
    } {
        \frac {
            b
        } {
            c
        }
    }
\stopformula

\startformula
    \frac {
        a
    } {
        \frac {
            b
        } {
            c
        }
    }
\stopformula

\stoptext
