Siivota is a verb. Verbs don't take partitive case endings. Finnish has 6 verb types, and verbs ending in -ota are type 4. When they're conjugated in present tense, the -ta is dropped, and an -a is added, leaving siivoa as the stem, to which the personal endings are attached when you conjugate. Siivoan, siivoat, etc. Same with haluta, pelata, and so on. Verbs ending in -ita and -eta are different types and conjugate differently.
Some type 4 verbs like tavata and pelätä undergo internal changes when conjugated.