Crossword Genius

Make a mess of Puck's anagram, putting the twelfth letter first (4,2)

Ross

I believe the answer is:

muck up

'make a mess of' is the definition.

I don't understand the rest of the clue.

(Another definition for muck up that I've seen is " Spoil (something)".)

I've seen this clue in The Guardian.
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