Extremely wicked but safe in our disguise (9)
I believe the answer is:
nefarious
'extremely wicked' is the definition.
(I've seen this in another clue)
'safe in our disguise' is the wordplay.
'disguise' indicates anagramming the letters (I've seen 'disguised' mean this).
'safe'+'in'+'our'='safeinour'
'safeinour' with letters rearranged gives 'NEFARIOUS'.
'but' acts as a link.
(Other definitions for nefarious that I've seen before include "immoral?" , "Extremely wicked" , "Wicked, criminal" , "Villainous" , "Very bad" .)