A policeman's gone with the cake decorator (7)
I believe the answer is:
officer
'a policeman's' is the definition.
('officer' can be a synonym of 'policeman')
'gone with the cake decorator' is the wordplay.
'gone' becomes 'off' (as in going off somewhere).
'with' says to put letters next to each other.
'the cake decorator' becomes 'icer' (I've seen this in another clue).
'off'+'icer'='OFFICER'
(Other definitions for officer that I've seen before include "Person appointed to position of responsibility" , "General, for example" , "One with commission" , "Person holding a commission in the services" , "Higher-ranking soldier" .)