Metal, extremely thick, insulating uranium (6)
I believe the answer is:
sodium
'metal' is the definition.
(I know that sodium is a type of metal)
'extremely thick insulating uranium' is the wordplay.
I cannot really see how this works, but
'extremely' could be 'so' ('so' can mean 'extremely' or 'very') and 'so' is located in the answer.
'uranium' could be 'u' (U is the chemical symbol for uranium) and 'u' is present in the answer.
'thick' could be 'dim' (I've seen this before) and 'dim' is found within the remaining letters.
No letters remain.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for sodium that I've seen before include "Soft metallic element" , "Chemical in salt" , "Common salt is ...... chloride" , "Soft metal, symbol Na" , "Soft, silvery-white metal" .)