The phrase "against the clock" means working or competing against a deadline, trying to complete something within a certain amount of time. Full definition
Race against the clock in Time Trials or collect various cupcakes hidden on each track. (gamesradar.com)
Working against the clock allows dogs to focus on the course and not on other animals. (eukanuba.com)
There are other options like the ability to race against the clock in time trials, but the real meat of the game will be played in those first three options. (engagedfamilygaming.com)