title: dplyr::select of nested data frame
body: <p>I have the following example data frame:</p>
  
  <pre><code>library(tibble)
  library(tidyverse)
  df &lt;- tibble(A = 1, B = 1)
  df2 &lt;- tibble(C = 2:4, D = 4:6)
  df &lt;- df %&gt;%
          nest(B) %&gt;%
          mutate(data = map(data, ~df2))
  </code></pre>
  
  <p>It's a nested <code>3x2</code> data frame (<code>df2</code>) in a <code>1x2</code> data frame (<code>df</code>). Is there a way to combine <code>purrr::map</code> and <code>dplyr::select</code> to select only <code>column C</code> in the nested data frame? <strong>I'm hoping to avoid <code>unnest</code>.</strong> The outcome should be:</p>
  
  <pre><code>      A             data
    &lt;dbl&gt;           &lt;list&gt;
  1     1 &lt;tibble [3 x 1]&gt;
  </code></pre>