Possible Clues:

• Do they get a cold at work?
• Assassins, in slang
• It's hard to be so cold as their job makes them
• NHLers
• I bond almost entirely with professional killers
• People who deliver frozen water
• Hard, cold hymns by the sound of them
• Ones with cool jobs?
• Hard work for them when it's cold
• Hard for them to be so cold