Is this the screw that holds the turn knob in place, covering the tap insert? It should be brass, which means that if you carefully drill the head off the turn knob should pull off. If your luck is in the remains of the screw could maybe be removed with pliers or Mole grips...if not, the whole insert can be unscrewed and replaced with a new one. If you do get the insert out, take it with you when seeking a replacement - there are a few different sizes.
John
July 2008