Hello,
I've got a question about this secondary mission (it's the one that awards Greedo to the imperial player). In this mission, you have to wound the marked hero, but in the scenario rules it is stated that, if the marked hero is adjacent to another rebel figure which is not wounded "Imperial miniatures can't declare attack targeting it" (I'm translating back to English from my Italian edition).

I've got a question about cleave: can I use it on the marked hero?
Cleave states that you can use it on miniatures that can be targeted from an attack, and the wording of the scenario spacial rule states that the attack cannot be declared... can it be done or not?

You're on the right track.

Cleave says to choose another figure or object that you could target for an attack. The mission rule prevents declaring the Top Target as a target of an attack when that hero is adjacent to another Rebel figure, so it also disallows choosing that hero for Cleave. (Blast would work, because you are not choosing a target figure or object for Blast.)