The term 'ATTACK ON A FLY' can be solved by the four-letter word 'SWAT'. SWAT stands for Special Weapons and Tactics, which is a law enforcement unit that specializes in high-risk operations such as hostage rescue, counter-terrorism, and riot control. The term is often used to describe a sudden, vigorous attack on an enemy or target. In this case, it could refer to a quick and decisive action taken against a fly.