Bryophytic growth (4)
I believe the answer is:
moss
'bryophytic growth' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for moss that I've seen before include "English racing driver, b. 1929" , "Small plant; peat bog" , "Plant forming most of my lawn!" , "Former racing driver" , "material not in Rolling Stones' collection?" .)