Paints generally are presented in complete systems. For example, Duplicolor's Engine Enamel system has a color component and a clear component. Some systems require a clear component, some don't have one, and in some a clear is optional.

Read the back of your paint can; if it says something about needing a clearcoat over it, then you'll know that your particular system does require a clear. Otherwise, if you want, you can go back to the store where you bought your paint and see if they have a "gloss clear" that you could use. Be aware, however, that the "gloss clear" may be identical to the paint you've already applied to your bike except that it won't have any pigment, and hence probably wouldn't give any greater resistance to chipping. Make sure to stay within the same system, because a different type of paint could potentially cause problems such as lifting or wrinkling the color coat.

Unfortunately, rattle-can paint is not as durable as professional-quality paints. You can search the forums for more information on painting your bike; it's a very common topic and you're likely to find more information than you'll ever need.