Malign a girl initially praised, turning nasty (9)
I believe the answer is:
disparage
'malign' is the definition.
(thesaurus)
'a girl initially praised turning nasty' is the wordplay.
'girl' becomes 'g'.
'initially' says to put letters next to each other.
'turning nasty' is an anagram indicator (letters corrupted or damaged).
'a' after 'g' is 'ga'.
'ga'+'praised'='gapraised'
'gapraised' with letters rearranged gives 'DISPARAGE'.
(Other definitions for disparage that I've seen before include "I grasped a change to belittle" , "Run down" , "Scorn" , "Belittled" , "Slight" .)