Hi Dylan

Short answer is with custom work.

The idea is to assign ids to all filter elements or at least to the first you want filtered on init

So try opening /sections/portfolio_section.php and modify line 69 (assuming you are running the latest version)

<li><a href="#" data-filter=".<?php echo $portfolio_cat->slug; ?>"><h5><?php echo  $portfolio_cat->name; ?></h5></a></li>

to

<li><a href="#" id="<?php echo $portfolio_cat->slug; ?>" data-filter=".<?php echo $portfolio_cat->slug; ?>"><h5><?php echo  $portfolio_cat->name; ?></h5></a></li>

then open /js/scripts.js and replace all occurrences of 

jQuery("#all").click();

with 

jQuery("#clients").click();