Knot that designer rearranged (12)

I believe the answer is:

straightened

'rearranged' is the definition.
(arranged to be straighter or neater)

'knot that designer' is the wordplay.
'knot' indicates anagramming the letters (letters are tangled up).
'that'+'designer'='thatdesigner'
'thatdesigner' with letters rearranged gives 'STRAIGHTENED'.

(Another definition for straightened that I've seen is " Put in order".)

I've seen this clue in The Telegraph.

Want a hint initially instead of a full solution? Install my app