Clue: Frog once found pincer movement by crab's head pernicious (6,8)
Answer: PRINCE CHARMING
Explanation: Definition: Frog once
PINCER C[rab] + HARMING. To be picky again, I don't think the frog that turns into a prince when kissed is actually Prince Charming (as in stories such as Cinderella or Sleeping Beauty)