What's the matter as lamp melts? (6)
I believe the answer is:
plasma
'what's the matter' is the definition.
(plasma is a kind of matter)
'as lamp melts?' is the wordplay.
'melts?' is an anagram indicator (I've seen 'melt' mean this).
'as'+'lamp'='aslamp'
'aslamp' is an anagram of 'PLASMA'.
(Other definitions for plasma that I've seen before include "Liquid part of blood" , "hot stuff" , "Blood component and type of television" , "Screens of some TVs" , "Star material" .)