Brie melted in air? That's serious (8)
I believe the answer is:
sobering
'serious' is the definition.
(similar in meaning)
'brie melted in air?' is the wordplay.
'melted' indicates an anagram.
'in' means one lot of letters goes inside another.
'air?' becomes 'song' (I've seen this before).
'brie' with letters rearranged gives 'beri'.
'beri' placed within 'song' is 'SOBERING'.
'that's' is the link.
(Other definitions for sobering that I've seen before include "serious" , "Causing to become thoughtful" .)