Seaweed dispersed by a gale (5)
I believe the answer is:
algae
'seaweed' is the definition.
(I've seen this before)
'dispersed by a gale' is the wordplay.
'dispersed' is an anagram indicator.
'by' means one lot of letters go next to another.
'a' put after 'gale' is 'galea'.
'galea' is an anagram of 'ALGAE'.
(Other definitions for algae that I've seen before include "Type of seaweed" , "Primitive organisms" , "Simple organisms, e.g. seaweeds" , "Simple plants, such as plankton" , "Scum" .)