Some flipping moss, a grass, or seaweed (8)
I believe the answer is:
'seaweed' is the definition.
'some flipping moss a grass' is the wordplay.
'flipping' is a reversal indicator.
'some' indicates the answer is hidden within the clue (some of the parts of the word are used).
'ossagras' is hidden in the letters of 'moss a grass'.
'ossagras' in reverse letter order is 'SARGASSO'.
'or' acts as a link.
(Other definitions for sargasso that I've seen before include "Gas soars around the North Atlantic sea" , "Region of Atlantic noted for masses of seaweed" , "Sea noted for seaweed and eels" , "Sea, weedy part of the Atlantic" , "Seaweed giving name to a sea" .)