Masses suspended at altitude (6)
I believe the answer is:
clouds
'masses suspended at altitude' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for clouds that I've seen before include "''I've looked at .... that way'' (Joni Mitchell)" , "They float in the atmosphere" , "obfuscates" , "Masses of vapour floating in the atmosphere" , "work by 3 [FRAYN] ?" .)