The term 'underhanded one' can refer to a variety of people who use unethical or dishonest tactics to get what they want. The crossword solutions for this term include CONNIVER (8 letters), sneaker (7 letters), briber (6 letters), WEASEL (6 letters), SNEAK (5 letters), and WIREPULLER (10 letters). A CONNIVER is someone who uses cunning and deceit to gain an advantage. A sneaker is someone who acts in a sly or sneaky manner. A briber is someone who offers money or other incentives to influence someone's decision. A WEASEL is someone who is untrustworthy and deceitful. A SNEAK is someone who acts in a stealthy or secretive way. Finally, a WIREPULLER is someone who manipulates people or events from behind the scenes.