Grass over pretty Cambridge Backs (3)
I believe the answer is:
rye
'grass' is the definition.
(I've seen this before)
'over pretty cambridge backs' is the wordplay.
'backs' indicates one should take the final letters (I've seen 'back' mean this).
The final letters of 'over pretty cambridge' are 'RYE'.
(Other definitions for rye that I've seen before include "Kind of serial or American whiskey" , "Grain crop" , "One of Cinque Ports" , "Gypsy gentleman" , "A Cinque Port; a grain" .)