That seems like a fairly powerful ability. I like it though. It makes sense for a person with a longer weapon to be able to engage an attacker first. Kind of the whole point of the weapon design.
I don't think tying it to bog standard 'level 1 spear' would be a good idea, but as a Hero trait, a Magic tree spear ability, looted magic items, certain monsters possessing it inherently (that can be recruited with civic tech), etcetc
Currently, multi-figure units suffer disproportionately from taking the first hit in combat, having something to counteract it would be nice.
I almost think multi-figure units should be unable to lose more than 1-2 figures per attack, barring certain special attacks or other multi-figure attacks, but that would be another too-major change most likely.