Sue changed completely, advantageously (8)
I believe the answer is:
usefully
'advantageously' is the definition.
(I've seen this before)
'sue changed completely' is the wordplay.
'changed' indicates anagramming the letters.
'completely' becomes 'fully' (I've seen this before).
'sue' anagrammed gives 'use'.
'use'+'fully'='USEFULLY'
(Other definitions for usefully that I've seen before include "Advantageously" , "quite profitably" , "In a productive manner" , "in an advantageous way" , "To produce a benefit" .)