The term 'Part of a Football Defense' can be summarized by the 10-letter solution 'Linebacker'. Linebackers are defensive players in American football who line up behind the defensive line and in front of the secondary. They are typically the most versatile players on the field, as they are expected to both stop the run and cover receivers. Linebackers are often the leaders of the defense, making calls and adjustments based on what they see from the offense.