Any time you are performing an API request on behalf of a user you need to use a user access token. In the case of create clip the docs mention: The new clip is marked as having been created by the user specified by the OAuth token .... which is why you need a user token and the appropriate scope.
A simple guide to follow is that if no token is required at all, then you can use an app access token to make use of increased Helix rate limits. If a token is required then it’ll be a user token with whatever scopes that endpoint needs.