Constable's disillusioned with sweet king (7)
I believe the answer is:
officer
'constable's' is the definition.
(police officer)
'disillusioned with sweet king' is the wordplay.
'disillusioned with' becomes 'off' (as in going off something).
'sweet' becomes 'ice' (ice is a kind of sweet).
'king' becomes 'R' (abbreviation for rex, king in Latin).
'off'+'ice'+'r'='OFFICER'
(Other definitions for officer that I've seen before include "One with commission" , "Holder of responsible position" , "Leading soldier" , "Holder of a commission in the armed forces" , "Police constable" .)