[LAWS]16.7 (c) [FONT=fs_blakeregular]When the ball has been clearly won by a team at a ruck and the ball is available to be played the referee will call "Use it!" after which the ball must be played within five seconds. If the ball is not played within five seconds the referee will award a scrum and the team not in possession of the ball at the ruck is awarded the throw-in.[/FONT][/LAWS]If time expires before the 5 seconds elapses (if there is an official stadium clock), then you can do nothing.
If there are still a few seconds to go, you can award a scrum, which will be played - Law 5.7 (e).
In the sticks, you will be the sole time-keeper.
For offside etc, you can play advantage. Not practical here.