grandfather clock

A grandfather clock uses a simple pendulum to keep an accurate track of time. The seconds hand of the clock moves forward exactly one second each time it moves forward from A to C, and another second each time it moves from C to A.
Determine the length of the pendulum in a grandfather clock that would accurately tell the time. The average gravitational field strength (g) at the Earth's surface is 9.81 ms^(-2).

In this case A and C are the points of maximum amplitude/displacement. So the period would be 2 seconds and the frequency would be 0.5Hz.

But how am I supposed to work out the length???

There is a formula for the period T of a pendulum in terms of its length L and g.
This formula must be in your notes or book.
Rearrange it to find L and sub the values you have for T and g