Claus the Saint can draw! 5 cards. He dies. When the Ghost Town is in effect, is he still allowed to come back and use his ability?
It depends on the character ability and in the case of Claus the Saint and other characters with beginning of turn draw abilities those abilities will not work.
Ghost Town: during their turn, killed characters return in play as ghosts, with 3 cards. They cannot be killed but they leave the game immediately after their turn.
The Ghost Town card specifies that the player returns with 3 cards which overwrites Claus the Saint's ability (and any other character with a beginning of turn draw ability)
There are characters that can use their ability during Ghost Town and here is an example.
Suzy Lafayette = As soon as she has no cards in her hand, she draws a card from the draw pile. (4 life points)
Yes he can use his ability.
For reference the Ghost Town card reads:
During their turns, eliminated players return to the game as ghosts. They draw 3 cards instead of 2, and cannot die. At the end of their turn, they are eliminated again.
Note that it states that they are to draw cards, so the ghost's will enter at phase 1, the draw phase.
Clause "The Saint"'s special abillity is:
He draws one card more than the number of players, keeps 2 for himself, then gives 1 to each player.
From the answer to Q14 on the FAQ for the High Noon from the publisher dV Giochi:
A. Bill Noface enters in play with 5 cards, Pixie Pete with 3 cards.
We can see that a players special ability does not suffer from the Ghost Town card being put into play. However as Claus "The Saint" is not specified in any FAQ that I could find, we do not know if he should have gotten the advantage that the Ghost Town card gives ("adding an extra card to the ghost", and thus giving him 6 cards to draw. So that he would end up with 3 cards for himself.), but seeing that Pixie Pete starts with 3 cards and not 4 (His special ability +1 (for the "extra card" from the Ghost Town), we have to assume that Claus "The Saint" get neither benefits or drawbacks from the the Ghost Town card.