Possible Clues:

• Use a code on a video game
• Unwanted casino guest
• One of various birds, including European rook
• Use marked cards, e.g.
• Compete dishonestly
• It isn't fair to give temperature in Celsius
• Act dishonestly (for gain)
• One fooling her heart in online activity
• It's not right for that cat to be around a 21 down cat
• Con man