This is an excellent question. You have plotted the glucose concentration versus the reciprocal of time (1/t) and gotten a straight line. You are right to be skeptical about calling 1/t the rate since a rate is change in concentration over time. We can think of the reciprocal of time as a sort of simplified stand in for the initial rate of the reaction since the initial rate is directly proportional to 1/t. So while you could call it the rate, it is probably best to keep it labelled as 1/t.
The importance of this graph is that is shows that the reaction is first order in respect to glucose. For a first order reaction, plotting rate (or in this case 1/t as a simplified stand in) versus concentration gives a straight line. Since the reaction is first order, you could also plot the natural logarithm (ln) of the concentration versus time to get a straight line. The slope of this line would be equal to -k (negative rate constant). Since the ln of a number is unitless, this means that the slope of the line would have the unit of 1/s, which is the correct unit for a first order rate constant.