diff --git a/ros2bag/ros2bag/verb/play.py b/ros2bag/ros2bag/verb/play.py
index c645de431..10fa1e0a4 100644
--- a/ros2bag/ros2bag/verb/play.py
+++ b/ros2bag/ros2bag/verb/play.py
@@ -89,7 +89,7 @@ def add_arguments(self, parser, cli_name):  # noqa: D102
                  'See storage plugin documentation for the format of this file.')
         clock_args_group = parser.add_mutually_exclusive_group()
         clock_args_group.add_argument(
-            '--clock', type=positive_float, nargs='?', const=40, default=0,
+            '--clock', type=positive_float, metavar='Hz', nargs='?', const=40, default=0,
             help='Publish to /clock at a specific frequency in Hz, to act as a ROS Time Source. '
                  'Value must be positive. Defaults to not publishing.'
                  'If specified, /clock topic in the bag file is excluded to publish.')