During Charity, the rules state to play or give away cards until there are only 5 in your hand. Can you play a monster card and fight it at this point?
From the rules "When To Play Cards" (page 5):
If drawn face-up, during the “Kick Open The Door” phase, they immediately attack the person who drew them. If acquired any other way, they go into your hand and may be played during “Looking For Trouble,” or played on another player with the Wandering Monstercard.
Monster cards can only be played during the "Kick Open The Door" phase or by other cards like Wandering Monster which explicitly allow them to be played. Thus you cannot simply play and fight them prior to charity.
Theoretically if you happened to also be holding a card that allowed you to play a monster as part of its effect, then you could play that card and the monster, depending on the conditions of the card. For instance, Wandering Monster itself doesn't help here as it is played on a player already in combat, but there could come along a new card someday (or there could be one in some expansion I don't have) that could allow it.
Playing and fighting are not the same thing. For example, Wandering Monster allows you to play a monster, but you don't get to fight it. Playing is to put on the table. Something must instruct that player to fight it if he is to fight it.
The Charity phase doesn't instruct any player to fight monsters played during Charity phase, so if you were to play a monster, you'd be left with a monster on the table that no one has a way of attacking. It would be stuck in limbo for the rest of the game. That doesn't make any sense, so playing a monster during charity doesn't either.
The Charity phase is simply giving you the chance to play cards before you have to discard, but you still have to follow the rules that cover how each type of card can be played. The section titled "When to Play Cards" details this. For monster cards, it says:
If drawn face-up, during the “Kick Open The Door” phase, they immediately attack the person who drew them.
If acquired any other way, they go into your hand and may be played during “Looking For Trouble,” or played on another player with the Wandering Monster card.