They are used for heating basic materials (8)
I believe the answer is:
elements
'they are used for heating' is the definition.
The definition suggests an adverb but the answer is not.
'basic materials' is the wordplay.
'basic' becomes 'element' (I've seen this before).
'materials' becomes 's' (I can't explain this - if you can you should believe this answer much more).
'element'+'s'='ELEMENTS'
Can you help me to learn more?
(Other definitions for elements that I've seen before include "Hydrogen, helium, ..." , "Basic parts" , "He and I are two" , "live wires?" , "Components or weather conditions" .)