A retainer is a fee paid to secure your services in the future (usually at short or little notice). Additionally, "Retainer" is basically a person who is available to you for advise or services. He is not required to put in daily attendance or observe any fixed working hours. He charges a retainership fee for his services.

In case, he is required to come and put in attendance or appear on your behalf before any authority, he charges you separately for such services. This however, depends upon the terms of his/her engagement as a 'Retainer'. So a "retainership", which isn't actually a real word, would be when you agreed to this and accepted the retainer.