Posh stuff to cocoon a worm (4)
I believe the answer is:
silk
'posh stuff' is the definition.
'silk' can be an answer for 'stuff' (silk is a kind of stuff). I am not sure about the 'posh' bit.
I can't explain the rest of the clue.
Can you help me to learn more?
(Other definitions for silk that I've seen before include "Fabric from a worm's cocoon" , "Fibre from moth larva" , "Item of fine legwear" , "Fabric made from fine threads of certain worm" , "Fabric from larvae" .)