In need of a content replace filter for posts in a specific wordpress category

Just put an in_category() conditional around your content-filtering code, wrap the whole thing in a function, and hook it into the the_content filter:

function mytheme_filter_cat_7_content( $content ) {
    if ( in_category( '7' ) ) {
    // filter your content here
    }
// then return $content
return $content;
}
add_filter( 'the_content', 'mytheme_filter_cat_7_content' );

(Just put whatever code you’re using to filter the_content inside the conditional.)

Also, note that you can pass either a category ID or slug to in_category().