In the reaction Mg+Cl2 gives MgCl2. Chlorine may be regarded as a reducing agent. How?
Reduction can be defined as
1) The addition of hydrogen
2) The removal of oxygen
3) Gain of electrons
The reaction between magnesium and chlorine is written as follows
Mg + Cl2 -------> MgCl2
Here chlorine gains the two electrons that are lost by magnesium. A reducing agent is a chemical substance that brings about the reduction of the other reactant, itself getting oxidised in the process. Similarly an oxidising agent is a chemical substance that brings about the oxidation of the other substance, itself getting oxidised in the process. Therefore, here chlorine is not the reducing agent but the oxidsing agent as it gains two electrons and brings about the oxidation of magnesium.