Clue: Zero tax e.g. extorted for portion of old ploughland (6)
Answer: OXGATE
Explanation: Zero tax e.g. extorted for portion of old ploughland (6)
***(o tax eg)** where the O = "zero" 
*An oxgate was an area of land that an ox could be expected to plough in a year (approximately 13 acres), more commonly known as an "oxgang".