What would a clause which permits modification look like?

Your lawyer can assist in writing the clause, but a typical one would read:

In the event that the husband’s income from all sources whether taxable or not is reduced through no voluntary act of the husband in any year to a figure less than 80% of the income for the year this agreement was signed, then thereafter, and for as long as this condition exists, the husband’s weekly payments to the wife shall be reduced to 80% of the weekly figure set forth elsewhere in this agreement.

Note that this clause provides a minimum amount below that may not be exceeded. Such a minimum amount would prevent your wife from becoming a public charge, which would be more likely to be accepted by the court.