Pre-grown grass (3)
I believe the answer is:
sod
'pre-grown grass' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for sod that I've seen before include "'Sward, turf (3)'" , "Surface layer of ground with grass roots" , "Piece of grassy soil" , "Piece of turf; a Law named for him" , "Lump of earth" .)