You could write a module that adds a twig extension to return the value of src from the string. That way you get to utilise PHP as per https://stackoverflow.com/questions/17017684/how-to-get-url-from-the-iframe-using-php to get the value.

https://pluginfactory.io/ is a good starting point for adding a module to Craft.

alternative very dirty approach: If the iframe markup is always going to be the same, you could do |replace(), replacing everything before the src=" and everthing after the " width with empty strings. Then replace " width with an empty string.