Warning posted in kitchen garden
I believe the answer is:
en garde
'warning' is the definition.
(I've seen this before)
'posted in kitchen garden' is the wordplay.
'posted in' indicates the answer is hidden within the clue.
'EN GARDE' is hidden in the letters of 'kitchen garden'.
(Other definitions for en garde that I've seen before include "Words of warning" , "(In fencing) in a defensive stance" , "Fencer's call" , "The opening position in fencing - mind yourself" , "Angered with opening position in fencing - be alert. (2,5)" .)