A successful comment will:
(1) solve the clue in the post or in a successful comment that has not yet been answered successfully
(2) supply an additional clue whose answer:
(3) crosses the previous answer; and
(4) if it crosses any other answers, does so legally.
Extra love for
(5) creating or maintaining symmetry
(6) advancing a theme consistent with clues solved so far
(7) puns like fingernails on a blackboard.