how to clear mould in basement?

A dehumidifier is a great idea, but if you have mold in the sheet rock, and it does not wash off with bleach/water OR peroxide/water - if it is inside the sheet rock, then the sheet rock has to come down and the inner walls dried, and new sheet rock replaced.

What you need most of all is good room ventilation.

But dont open doors or windows when its warm outside, the warm air will cause more humidity on the assumingly cold basement walls.

The mould has to be dealt with or it will spread!

Best solution is spraying/washing it with clor disinfectants (try clorox or mixtures with about 5% clor).

This will also prevent a possible return of the mould.

Redo the sheetrock. Not many other alternatives. There will be moisture trapped behind the walls. More so if you have insulation. Open up the walls, replace the insulation if you have any. Check for wood rot to the framing, if you have any. Let it dry out, and install new sheetrock, joint compound, and repaint.

try bleach(spray bottle/with water60/40 and lots of ventalation), but go back and look about 2 weeks later inbehind the sheetrock wall if it keeps growing will eat at the wood

You should try vinegar and salt.You can spray and wipe.But always handle mold with care.Never let your skin touch it.Use always rubber gloves.This suggestion about spraying is just a tip that I have tried my self and was successful.there come definitely expert solutions.

bleach, a dehumidifier, fans

Call a mold guy, he should know what to do... Do they have mold guys, I might have just made that up.

a good bleach water solution should help