Possible Clues:

• Person who crosses the line?
• Counsel the sailor, one who crosses the line
• Target of picketers' epithets
• Opera cuts to bite, initially, then sign of wound healing
• It's hard to cover the wound - caused by the strike, perhaps
• Dried crusty surface of a healing wound
• Line-crosser?
• Not strikingly hard on top
• Crust over sore or strike-breaker
• Natural wound cover