The phrase 'the clock is ticking' refers to time moving or running out in order to be able to do something or to avail of an opportunity. If you remind someone 'the clock is ticking' you are informing them that there is a limited period of time in which to do something. This expression is often used to talk about women's 'biological clocks' ticking or the realisation that time is running out for them to have a child if they wish.

1. There's still time for the team to win the game, but the clock is ticking.

1. The clock is ticking on the order. If we don't have it completed by the end of the day, then we will miss the deadline.
2. The police have made us aware that the clock is ticking. We have eight hours to charge him with something before we have to set him free.