Cheese gathering is most fleeting (8)
I believe the answer is:
briefest
'most fleeting' is the definition.
Both the answer and definition are superlative adjectives. Perhaps you can see an association between them that I can't see?
'cheese gathering' is the wordplay.
'cheese' becomes 'brie' (Brie is an example).
'gathering' becomes 'fest' (I can't explain this - if you can you should give a lot more credence to this answer).
'brie'+'fest'='BRIEFEST'
'is' acts as a link.
Can you help me to learn more?
(Other definitions for briefest that I've seen before include "Most perfunctory" , "over most quickly?" , "Extremely short" , "Least lengthy" , "Of shortest duration" .)