Ephemeral thing could work? (6)
I believe the answer is:
mayfly
'ephemeral thing' is the definition.
Although both the answer and definition are singular nouns, I cannot see how they can define each other.
'could work?' is the wordplay.
'could' becomes 'may' (I've seen this before).
'work?' becomes 'fly' (I can't explain this - if you can you should give a lot more credence to this answer).
'may'+'fly'='MAYFLY'
Can you help me to learn more?
(Other definitions for mayfly that I've seen before include "Short-lived spring insect" , "Ephemeral insect" , "Short-lived insect with delicate wings" .)