Thermal rating number: one (8)
I believe the answer is:
together
'one' is the definition.
Both the definition and answer are adjectives. Maybe there's a link between them I don't understand?
'thermal rating number' is the wordplay.
'thermal rating' becomes 'tog' (I can't explain this - if you can you should give a lot more credence to this answer).
'number' becomes 'ether' (anaesthetic, something which numbs).
'tog'+'ether'='TOGETHER'
Can you help me to learn more?
(Other definitions for together that I've seen before include "Got there (anag.)" , "In company or in conjunction" , "Concertedly" , "Collected" , "without a break?" .)