Can I teach learner to drive with third party, fire and theft cover on my car?

If your wife is going to be sitting in the driver's seat and take control of the car while you supervise her, then she'll need to be insured as well. From what you wrote, it seems that you have a car insurance policy just for yourself, so you'll have to talk to your current insurer to see if they can add your wife to the policy as well.

If you do not want to do that, then you can look into short term learner insurance which will provide cover while you are teaching her to drive. It might be more appropriate to do that though as you can get her insured only for the duration of her learning (subject to a minimum of 4 weeks) rather than being stuck with a yearly contract.