Structure: fine thing carelessly dropped, briefly (6)
I believe the answer is:
fabric
'structure' is the definition.
(I know that fabric is a type of structure)
I can't explain the rest of the clue.
Can you help me to learn more?
(Other definitions for fabric that I've seen before include "Building structure; textile" , "Underlying structure" , "Perhaps felt" , "handiwork?" , "(Texture of) cloth" .)