St Clair served as honorary secretary to Winston Churchill from 1948 to 1950,[1] before returning to run his family's dairy farm at Tetbury in Gloucestershire.

At the 1959 general election he stood as Conservative candidate in Bristol South East, but he lost to the sitting Labour Member of Parliament Tony Benn, whose majority was nearly 6,000 votes.

Benn, who wished to be allowed to disclaim his peerage, defied his inability to sit in the Commons by standing at the election, and he and St Clair were the only two candidates.

St Clair's campaign displayed posters near every polling station warning voters that Benn was disqualified and that any votes for him would have no effect.

)[4] Outside Parliament, Benn continued to campaign for a change in the law to allow him to disclaim his peerage and return to the Commons, and eventually the Conservative government agreed.

St Clair had already given an undertaking that he would respect the wishes of the people of Bristol South East if Benn became eligible to take his seat again, so he immediately resigned by accepting the office of Steward of the Manor of Northstead.