That clears it up pretty well. I actually prefer the attack to only work against a monster that is specifically attacking the spear wielder. I wouldn't have the bonus attack trigger against a monster that moves in range of the spear but either attacks another Hero or simply doestn't make an aggressive move against the spear weilder. Attacking safely from a diagonal is enough of a benefit. Having a spearman's shield in this case doesn't justify an extra attack (for me) if the monster doesn't aggressively move at the spear wielder protected by the specialty shield. I think the spearman's shield then reverts to a basic shield in this case, allowing no special ability.
I would even prefer there wasn't a bonus attack. This feels overpowered to me (though the ability doesn't always function, which helps). Seeing as a skull is likely, and most monsters will only sometimes defend it, I think allowing the attack to go early like an ambush is already an effective benefit. A dead monster can't attack back, so this magnifies the usefulness of the defensive benefit significantly. I'm mainly thinking the 1 Body Point monsters here, but getting in an extra Body Point early can mean a kill against multiple BP monsters in the Hero's turn as well.
I also think extra attacks could be abused with door-guarding. [edit] Playtesting would be more useful than my conjectures, however. I have been shown to miss the mark a bit in the past, and maybe I'm just getting reactionary.
For your consideration, here's a reworded alternative:
The Spearman's shield gives you
one extra combat die in defence.
If you are also using a spear and a
monster moves into an adjacent
square to attack you, you may use
your next action immediately to
attack it first.
Cost: 250 Gold Coins
May not be used by the Wizard.
Shield
I'm starting to think this weaker ability could work for the spear alone. I may have to steal it. [edit]