Tin on top of long grass (6)
I believe the answer is:
snitch
'grass' is the definition.
(both mean to inform on or betray)
'tin on top of long' is the wordplay.
'tin' becomes 'sn' (Sn is the chemical symbol for tin).
'on top of' means one lot of letters go next to another.
'long' becomes 'itch' (I've seen this before).
'sn'+'itch'='SNITCH'
(Other definitions for snitch that I've seen before include "Inform on, tell tales" , "Grass" , "I'm telling!" , "Steal; inform (on someone)" , "who tells on his classsmate?" .)