Is it a routine for Clear to disconnected the service for 10 - 20 minutes

As a former clear user, I can tell you I've encountered this problem many times over the two years I've had the service as well as many more. Clear is NOT DSL, it does NOT have a solid link to a phone line or anything like that. It's completely wireless-based. Which means it's subject to random outages, failures, and other unforseen complications. It's not uncommon (espically during hevey moments of traffic) for the connection to die and for a signal to be reestablished. Due to my location this would happen for hours at a time. Clear has been nothing but one problem after another. At least for me. And, it can take weeks for someone to come repair a problem. Half the team don't know what they're talking about, and they've been changing phone numbers and service. Needless to say Last year I decided I wasn't going to put up with it anymore. Personally... I'd advise you to find something a little more... stable.

If it happens everyday it means that your ISP is disconnecting it often, which means either a routine maintenance or a deliberately bad service. Maybe you can ask them directly about the issue, as unless specifically noted, your connection should be active at all times without any interruptions for an indefinite amount of time if you want to.

Our internet connection in the flat is i think as terrible as this that we always consider having it cut.

You can see it all the day as sometimes due to some connections the DSL connection is off. The best is to consult your ISP (internet service provider). Have a nice day!