Inform on the lawn? (5)
I believe the answer is:
grass
This clue is a double definition.
'inform on' is the first definition.
(I've seen this before)
'the lawn?' is the second definition.
(I've seen this before)
(Other definitions for grass that I've seen before include "German author" , "shop" , "Soporific" , "Snitch (sl.)" , "blow the gaff" .)