I'm self-employed, am I allowed to engage staff eg a clerk or a secretary?

You are allowed to take on staff, that is, recruit people to work for you but you should register yourself as an employer first. By law, you are required to notify the relevant authorities within 14 days of becoming an employer, otherwise you will be liable to a fine of up to Rs5,000. You will therefore have to complete a PAYE Employer Registration Form (ERF) which will give you access to your Employer Registration Number.

Source : MRA

The only exception is when you hire only household employees.

Note As an employer, you will be required to pay towards National Pensions Scheme (NPS) of your staff. Usually that's 6% (standard rate) of the wage you're paying to the person you've employed. You will also need to contribute towards the National Savings Fund (NSF), which is 2.5% of the basic salary and a Training Levy of 1% as well.

See this link for more information.

Yes. You become an employer. You will have the normal employer duties. Take a look at this.