Cirrus or Cumulus (5)
I believe the answer is:
cloud
This clue is a double definition.
'cirrus' is the first definition.
(I've seen this in another clue)
'cumulus' is the second definition.
(I've seen this in another clue)
(Other definitions for cloud that I've seen before include "Make less clear" , "Nebula" , "Mass of vapour visible in the air" , "Mass of water droplets" , "Visible vapour in sky" .)