Seaweed turned into moss — a grass (8)
I believe the answer is:
sargasso
'seaweed' is the definition.
(I've seen this before)
'turned into moss a grass' is the wordplay.
'turned' says the letters should be written backwards.
'into' indicates a hidden word.
'ossagras' is hidden within 'moss a grass'.
'ossagras' reversed gives 'SARGASSO'.
(Other definitions for sargasso that I've seen before include "Mass of weed" , "Atlantic sea surrounded by ocean currents" , "Sea known for seaweed" , "Sea with much floating weed" , "'Sea in the North Atlantic, the only sea without shores (8)'" .)