Pons-Collins:
fall guy - (esp US inf: = victim) armes Opfer, Angeschmierte(r) mf (inf); (= scapegoat) Sündenbock

Oxford-Duden:
fall guy (coll.) - a: (victim) Lackierte (salopp); b: (scapegoat) Prügelknabe

That doesn't seem completely right. The fall guy is the person set up by others to fall into the trap, to take the blame for everyone. It's not necessarily that he is inherently naïve or clueless, more that in a particular situation others have conspired to keep him out of the loop.

William Pepper, James Earl Ray's attorney, is seeking permission to test the murder weapon, which was found with James Earl Ray's fingerprints. Pepper and Ray, who contends he was a fall guy for the real killers, believe tests on the rifle will prove it was not used in the slaying. Prosecutors say the tests aren't warranted and could damage the evidence.

Ok, I searched for another example. And you are right, this has more the
connotation of a victim.

And it proves that Wordnet is not a reliable source (you once mentioned this
to me!). The patsy, though, might fit my first example.

From WordNet (r) 2.0 :
fall guy
n : a person who is gullible and easy to take advantage of [syn:
chump, fool, gull, mark, patsy, sucker, soft
touch, mug]