Submarine dunes are dispersed (8)
I believe the answer is:
undersea
'submarine' is the definition.
(submarine means under the sea)
'dunes are dispersed' is the wordplay.
'dispersed' is an anagram indicator.
'dunes'+'are'='dunesare'
'dunesare' anagrammed gives 'UNDERSEA'.
(Other definitions for undersea that I've seen before include "Submarine" , "Where bed is" , "Beneath the surface of the ocean" , "Beneath the waves" , "Beneath the oceans" .)