Etna's insides become black, containing cold material thrown out (6)
I believe the answer is:
ejecta
'material thrown out' is the definition.
(material forced out by a volcanic eruption)
'etna's insides become black' is the wordplay.
'etna' becomes 'eject' (I can't explain this - if you can you should believe this answer much more).
'insides' says to take the centre (I've seen 'inside' mean this).
'become black' becomes 'black' ('black' can be a synonym of 'become black').
The centre of 'black' is 'a'.
'eject'+'a'='EJECTA'
'containing cold' is the link.
This may not be correct. Some or all of it may be part of another bit of the clue.
(Other definitions for ejecta that I've seen before include "Stuff thrown" , "Material thrown out by volcano; ace jet (anag.)" , "Volcanic matter" , "matter being thrown out" , "Material forced out by an eruption" .)