Engagement letters - Clauses for cessation of your practice

I'm a new business that is preparing a letter of engagement for a new client.

I have worked in practice for over 12 years before going alone and have never had this come up before despite the thousands of engagement letters I have prepared for my numerous bosses over the years.

My prospective client has asked for a clause in case I decide to cease trading that I stay on until he finds a suitable replacement. I don't have an issue on this at all, its prudent for the client to want to feel he is covered as I am a very new business but wondered if anyone else had come across this before and how they would word such a clause?

Any help or opinion  greatly appreciated.

Martin