Cheerful fellow consumed brownie (4)
I believe the answer is:
airy
'cheerful' is the definition.
(thesaurus)
I don't understand how the rest of the clue works.
Can you help me to learn more?
(Other definitions for airy that I've seen before include "Well ventilated; nonchalant" , "Delicate; dismissive" , "Spacious - aerial" , "Delicate, casual" , "Lacking substance" .)