What will be the substitution product formed when AgCN reacts with an alkyl group? 

Alkyl isonitrile

Groups like cyanides and nitrites possess two nucleophilic centres and are called ambident nucleophiles. Actually cyanide group is a hybrid of two contributing structures and therefore can act as a nucleophile in two different ways [^-CΞN ↔ :C=N^-], i.e., linking through carbon atom resulting in alkyl cyanides and through nitrogen atom leading to isocyanides or isonitriles.