Set off in clearness, then they go up in flames (7,8)
I believe the answer is:
chinese lanterns
'they go up in flames' is the definition.
(floating lanterns containing flames)
'set off in clearness then' is the wordplay.
'set off' is an anagram indicator.
'in'+'clearness'+'then'='inclearnessthen'
'inclearnessthen' anagrammed gives 'CHINESE LANTERNS'.
(Other definitions for chinese lanterns that I've seen before include "They could brighten things up" , "products of Asian light industry?" , "Plants" .)