One of the rarest of virtue names, Clemency could come back along with the more familiar Puritan virtue names such as Hope and Faith. It has a rhythmic three-syllable sound, and offers a more virtuous alternative to the more popular Clementine.
Clemence is the medieval French form, also heard occasionally in the UK. Writer Clemence Dane was born Winifred Ashton.
Clemmie makes a cute nickname.