San Francisco Mayor Dianne Feinstein suffered the first veto override in the 6 1/2 years she has been in office when the Board of Supervisors voted to go ahead with an $8.8-million comparable worth plan. The mayor, who vetoed the measure last week, said she objected to paying the money to 7,000 city workers in the form of a $5-a-day meal allowance. She said the money represented a fringe benefit that, by law, only voters can approve. Supervisor Doris Ward told the members that the vote on the veto was a chance to "put your money where your mouth is." The mayor said she will ask voters in November to repeal the comparable worth package and prohibit supervisors from taking similar action in the future. Comparable worth calls for basing salaries on the amount of responsibility and training involved, as opposed to the traditional marketplace concept.