I'm Clerk/Governance Professional for 5 schools, I've also been a governor at various schools for 25 years. Like all jobs - there are pros and cons. In no particular order:
It very much depends on the school and people you're working with. I've been treated like royalty and given applause, chocolate and wine regularly, have also been treated like something on the bottom of your shoe and had orders regularly barked at me in front of other people. It is a female-dominated role so sexism plays a part; ironically, the worst treatment I've had has been from female heads and bursars.
If you get a good GB and head, who are well-organised and you are paid properly, it's a very rewarding and fascinating role. I honestly love it, most of the time.
It fits very well around other roles. If you already work in a school you have a head start especially with jargon. If your school job and clerk job are in the same school, you have to be very careful not to blur boundaries or allow others to do this. This is especially the case with panels (e.g. parent complaint, staff grievance) when senior staff might apply pressure - it is really crucial to maintain impartiality even though the head is your boss.
The role is generally under-recognised and underpaid considering you're meant to be a legal adviser. There is a campaign going on by the NGA to change this but it's very slow progress. They recommend at least £13ph I think. In my area, the local authority refuses to take any notice of this and the pay is around £12, and also the allocation of hours is absurdly low. So you end up working for nothing.
There is a huge shortage of clerks and turnover is high - partly because of the nature of the role (part time, very few hours) but also because it is underpaid and in at the deep end. In my area new clerks get 3 hours training. However, this does mean that clerks can be choosy about who they work with. I'm indundated with offers of work, to some extent I can name my price and it can be pretty lucrative.
Local authority support can be very patchy. Departments are often staffed with people including managers who have never done the job. There are a lot of grey areas in governance where you need advice from someone who's been there and can give pragmatic suggestions.
External support organisations like The Key and the NGA are generally amazing but access to them depends whether your school has subscribed. Depending on whether your school is maintained or an academy, you need to be very familiar with the regulations (maintained) or the Articles of Association (academies) and other academy legal docs.
If you can spare the time, being a governor at another school as well is fabulous CPD but this is unpaid.
Let us know what you decide!