hook_field

I would like to implement extra functionality on specific CCK fields operations. For example, I have a link CCK field in some content type. When a value, eg a website link, is entered in that field, I automatically want to set up a feed using that link. To do this, I am now altering the entire node form using hook_nodeapi(), filtering out the correct content type and then reading the field value. I thought it would be nicer to hook up my custom action (creating the feed from the link) directly to the CCK field.