Vacant sign seen on long grass (6)
I believe the answer is:
snitch
'grass' is the definition.
(both mean to inform on or betray)
'vacant sign seen on long' is the wordplay.
'vacant' means to remove the middle letters.
'seen on' says to put letters next to each other.
'long' becomes 'itch' (I've seen this before).
'sign' with its middle taken out is 'sn'.
'sn'+'itch'='SNITCH'
(Other definitions for snitch that I've seen before include "Whistleblower (informal)" , "I'm telling!" , "Inform on someone in slangy way" , "Police informer" , "Telltale (sl.)" .)