Act out of character, curiously more mischievous (6)
I believe the answer is:
archer
'more mischievous' is the definition.
(I've seen this before)
'act out of character curiously' is the wordplay.
'out of' is a deletion indicator.
'curiously' indicates an anagram.
'character' with 'act' taken out is 'charer'.
'charer' is an anagram of 'ARCHER'.
(Other definitions for archer that I've seen before include "A bow-and-arrow man" , "Toxophilist" , "One uses bows and arrows" , "stars" , "Jeffrey, bowman?" .)