Cumulus, say, could be dispersed (5)
I believe the answer is:
cloud
'cumulus say' is the definition.
(cumulus is a kind of cloud)
'could be dispersed' is the wordplay.
'be dispersed' is an anagram indicator.
'could' anagrammed gives 'CLOUD'.
(Other definitions for cloud that I've seen before include "Mass of vapour visible in the air" , "Condensed air vapour" , "Dull spot" , "Mass of airborne water vapour" , "Cover" .)