### Clue: Mutton dressed as lamb? That's Prince Charles (5,9)
### Answer: YOUNG PRETENDER 
 ### Explanation: Definition: That's Prince Charles
  Cryptic defn.  The idiomatic description is, usually disparagingly, used to describe an aging woman (a figurative comparison to mutton from an old sheep) dressed, and hence trying to pass off,pretending to be, young (a comparison to a young lamb).  Me, I don't think that anything disparaging can or should be said about those who can pull it off, certainly not the examples below.
Found this extract from a 1911 social gossip journal involving another prince:
"Some one the other day asked the Prince of Wales at the Ancient Music whether he did not think some girl pretty. 'Girl!' answered he, 'Girls are not to my taste. I don't like lamb; but mutton dressed like lamb!'."
The answer refers not to the current English prince, but Charles Stuart, Bonnie Prince Charlie, of "The Skye Boat Song" fame.  He was the "Young Pretender" to the English throne, to distinguish him from his father, who was also a pretender to the throne.