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 to get the value. 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.