Put on costume, turning arrogant (12)
I believe the answer is:
contemptuous
'arrogant' is the definition.
(thesaurus)
'put on costume turning' is the wordplay.
'turning' indicates anagramming the letters.
'put'+'on'+'costume'='putoncostume'
'putoncostume' with letters rearranged gives 'CONTEMPTUOUS'.
(Other definitions for contemptuous that I've seen before include "Condescending" , "Scornful" .)