Old official desirable? No sir! Corrupt! (6)
I believe the answer is:
beadle
'old official' is the definition.
(historical type of parish official)
'desirable? no sir corrupt' is the wordplay.
'no' suggests deleting specific letters.
'corrupt' indicates an anagram (a corrupted spelling of the word).
'desirable' with 'sir' removed is 'deable'.
'deable' anagrammed gives 'BEADLE'.
(Other definitions for beadle that I've seen before include "College official" , "University employee" , "Parish official (hist.)" , "College officer" , "Mr Bumble's position in "Oliver Twist"" .)