The term 'CAPTAIN's Underling' refers to two different positions in a maritime setting. The first is the LIEUTENANT, which is a 10-letter word. This is a military rank that is typically held by a commissioned officer who is subordinate to the CAPTAIN of a ship. The second is the MATE, which is a 4-letter word. This is a position held by a sailor who is responsible for the day-to-day operations of the ship and is subordinate to the CAPTAIN. Both of these positions are essential to the successful operation of a vessel.