Furtive air at reorganised administrative department (11)
I believe the answer is:
secretariat
'administrative department' is the definition.
(I've seen this before)
'furtive air at reorganised' is the wordplay.
'furtive' becomes 'secret' (I've seen this in another clue).
'reorganised' indicates an anagram.
'air'+'at'='airat'
'airat' anagrammed gives 'ariat'.
'secret'+'ariat'='SECRETARIAT'
(Other definitions for secretariat that I've seen before include "Admin area" , "Administrative office" , "administrative department" , "Office staff" , "Admin department" .)