Rock growth (4)
I believe the answer is:
moss
'rock growth' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for moss that I've seen before include "Supermodel Kate, b.1974" , "Plant(s) not wanted on lawns" , "Plant forming most of my lawn!" , "Simple green plant" , "Small flowerless green plant without proper roots" .)