Rat on the lawn (5)
I believe the answer is:
grass
I believe this is a double definition.
'rat' is the first definition.
(I've seen this before)
'the lawn' is the second definition.
(I've seen this before)
'on' acts as a link.
(Other definitions for grass that I've seen before include "Police informer; plant" , "blow the gaff" , "Dope" , "Common plant" , "Turf" .)