Possible Clues:

• Doesn't play by the rules
• 'The century is warming up for 4 across, for instance (6)'
• Has an ace up one's sleeve, say
• Looks at the answers beforehand, e.g.
• Chaste (anag.)
• Peeks at a classmate's paper, say
• Acts dishonestly, perhaps in card game
• The century is not fairly warming
• It gets not fairly warm when one takes the top off
• How the century gets not fairly warm