If it’s just needed in one place, you could use the following:
<?php echo wp_strip_all_tags( get_the_excerpt( ) ); ?>
Keep in mind, using the custom excerpt field will automatically remove all tags. Creating an excerpt automatically from the post content will not.
If you want more control, you can use wp_trim_excerpt() or wp_trim_words().
If you need more control to add something like this:
<?php
echo wp_strip_all_tags( wp_trim_words( get_the_excerpt(), 60, '[...]' ) );?>
wp_trim_words() allows you to pass in the text source, number of words, and the more text. You can remove the more text by setting this to false.