It sounds like all you wanna do is cut out the banana (element b). This is the same whether or not it's on top of ANYTHING else - you're cutting a foreground object out - what (if anything) is underneath doesn't matter.

This would be using any or all (in combination - particularly if you're dealing with a shallow DOF) of the following - Paths, Lasso Tool, and Masking.

Can you post a pic of the actual image you're working on?
-Lew ;-)