The figure below shows how to build NOT, OR and AND gates from a single MUX.

Next in line we have to somehow build the flipflop in the circuit. We could build a latch from a single MUX quite easily if we feedback the output to one of the MUX inputs. The figure below will make everything clearer. Notice that we could easily construct a latch which is transparent while its clock input is high or low by just changing the input the feedback wire is connected to.
We then use two latches, one transparent low the other transparent high to construct a flipflop.

As a final note, some use the versatility of the MUX structure to their advantage by spreading MUX structures as spare cells. Later if an ECO is needed one can build combinational as well as sequential elements just from those single MUX structures.

Your company is pretty tight on budget this year and it happens to have only Muxes to design with.
You are required to design a circuit equivalent to the one below, using only Mux structures.

Given only two xor gates one must function as buffer and another as inverter? 

Tie one of xor gates input to 1 it will act as inverter. 
Tie one of xor gates input to 0 it will act as buffer.