Lawn material (3)
I believe the answer is:
'lawn material' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for sod that I've seen before include "A lump of turf (3)" , "Piece of peat or grass" , "A lulmp of turf" , "Surface layer of ground with grass roots" , "A piece of sward." .)