Unlikely material for slippers, that's clear (5)
I believe the answer is:
glass
'unlikely material' is the definition.
'glass' can be an answer for 'material' (I have seen 'Vitreous material ' mean 'glass' so perhaps 'material' could also mean 'glass'). I am unsure of the 'unlikely' bit.
I can't explain the remainder of the clue.
Can you help me to learn more?
(Other definitions for glass that I've seen before include "measure of wine" , "Balloon, say" , "Container for drinking from" , "Drinking vessel" , "Transparent solid" .)