Here in board meeting a knight starts to stiffen (4)

I haven't cracked this one yet — but the brilliant team at Fifteensquared have:

In chess (a meeting of two players at a board), one of the knights starts in square B1 = B ONE in algebraic chess notation. Bone, as a verb = to stiffen a garment (typically a corset or bodice) with long thin strips, originally made of whalebone but now usually metal or plastic.