Possible Clues:

• Not consistent or having a fixed pattern
• X or Y, in algebra
• With no fixed pattern
• Fickle mathematician's expression
• Opposed to agricultural land in which 'i' is used for 'x'?
• Unstable British settling east of drowned valley, in another valley
• In the answers to 17-, 34- and 57-Across, it was replaced in turn by 1, 2, 3, 4, 5 ...
• Changeable, so changeable, very changeable
• Algebraic uncertainty
• Not consistent