Clanrats are the under-empire made visible: an endless, chittering tide of spear-clutching vermin driven up from the burrows whenever a warlord needs bodies. Individually they are scrawny, skittish, and armed with whatever rusted castoffs the clan could spare; collectively they are a flood that has drowned fortress cities one expendable body at a time.
A clanrat fights because the whip is behind him, the press of his kin is beside him, and somewhere far to the rear his warlord has promised food to the survivors. Doctrine among the clans holds that any battle can be won if enough clanrats are spent on it, and skaven history — written by the warlords, never by the clanrats — largely bears the doctrine out.