A basket full of fruit in which a rock fell [not a basis, look at the next section] is not muktza as the fruit is more important then the rock. If you need the space the basket is on, you can pick up the whole basket and put it where you want. If you don’t want the space, just the fruit, you can only move the basket if by emptying out both the fruit and the rock, the fruit will get ruined.

Chapter 309 Section 4-5

‘Basis ledavar haasur’ means a base for muktza. Chazal said that not only the object of muktza cannot be moved, but also if you put the object on something else with the intent to leave it there through ‘ bain hashimashos’ [twilight] of Shabbos, that something else is a base and also becomes muktza. This base continues to be muktza even if the real muktza was taken off on Shabbos.

If a rock was put on a pot [not specifically for Shabbos], even though the pot does not become a ‘base’, still the pot cannot be moved. You can shake the pot to get the rock off, if you want what is in the pot. Another way would be to knock the rock off with your body, then it is permitted even if you do not want what is in the pot.