[youtube:clips] Support downloading clips

Closes #2543

yt_dlp/extractor/youtube.py

@@ -5978,14 +5978,43 @@ class YoutubeTruncatedURLIE(InfoExtractor):
             expected=True)
 
 
-class YoutubeClipIE(InfoExtractor):
+class YoutubeClipIE(YoutubeTabBaseInfoExtractor):
     IE_NAME = 'youtube:clip'
-    IE_DESC = False  # Do not list
-    _VALID_URL = r'https?://(?:www\.)?youtube\.com/clip/'
+    _VALID_URL = r'https?://(?:www\.)?youtube\.com/clip/(?P<id>[^/?#]+)'
+    _TESTS = [{
+        # FIXME: Other metadata should be extracted from the clip, not from the base video
+        'url': 'https://www.youtube.com/clip/UgytZKpehg-hEMBSn3F4AaABCQ',
+        'info_dict': {
+            'id': 'UgytZKpehg-hEMBSn3F4AaABCQ',
+            'ext': 'mp4',
+            'section_start': 29.0,
+            'section_end': 39.7,
+            'duration': 10.7,
+        }
+    }]
 
     def _real_extract(self, url):
-        self.report_warning('YouTube clips are not currently supported. The entire video will be downloaded instead')
-        return self.url_result(url, 'Generic')
+        clip_id = self._match_id(url)
+        _, data = self._extract_webpage(url, clip_id)
+
+        video_id = traverse_obj(data, ('currentVideoEndpoint', 'watchEndpoint', 'videoId'))
+        if not video_id:
+            raise ExtractorError('Unable to find video ID')
+
+        clip_data = traverse_obj(data, (
+            'engagementPanels', ..., 'engagementPanelSectionListRenderer', 'content', 'clipSectionRenderer',
+            'contents', ..., 'clipAttributionRenderer', 'onScrubExit', 'commandExecutorCommand', 'commands', ...,
+            'openPopupAction', 'popup', 'notificationActionRenderer', 'actionButton', 'buttonRenderer', 'command',
+            'commandExecutorCommand', 'commands', ..., 'loopCommand'), get_all=False)
+
+        return {
+            '_type': 'url_transparent',
+            'url': f'https://www.youtube.com/watch?v={video_id}',
+            'ie_key': YoutubeIE.ie_key(),
+            'id': clip_id,
+            'section_start': int(clip_data['startTimeMs']) / 1000,
+            'section_end': int(clip_data['endTimeMs']) / 1000,
+        }
 
 
 class YoutubeTruncatedIDIE(InfoExtractor):