Gimba

Hello, Can I claim cleaning allowance whilst working for royal mail full time, and having my own "self employed" job selling on ebay? when I go to claim, it asks if i've done a self assessment, which I have, and then goes on to say i need to claim through that. But my self assement part has nothing to do with working for royal mail, and its the royal mail uniform i'm claiming for, I don't wear this when doing my 2nd side little money earner through ebay thanks