Terrarium growth (4)
I believe the answer is:
moss
'terrarium 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 "Shape of pointed arch" , "A rolling stone gathers none" , "Small flowerless green plant without proper roots" , "It carpets moist shady places" , "English racing driver, b. 1929" .)