Possible Clues:

• __ the line
• ___ the line (obeying orders)
• Angling in, as a nail
• ___ the line (obeying)
• Pulling, we hear, on the line -- or standing by?
• Conforming to
• Driving a nail obliquely
• -- the line (conforming)
• How 21 across might be giving one a tug, by the sound of it
• ..... the line is conforming obediently