Fireball more disastrous, full of energy and temperature (6)
I believe the answer is:
meteor
'fireball' is the definition.
(thesaurus)
'more disastrous full of energy and temperature' is the wordplay.
'disastrous' indicates an anagram.
'full of' means one lot of letters goes inside another.
'energy' becomes 'e' (physics symbol as in E=mc2).
'and' means one lot of letters go next to another.
'temperature' becomes 't' (abbreviation).
'more' is an anagram of 'meor'.
'e' after 't' is 'te'.
'meor' enclosing 'te' is 'METEOR'.
(Other definitions for meteor that I've seen before include "lump of rock" , "Glowing heavenly body" , "Space traveller" , "it appears as a streak of light" , "Remote sort of heavenly body" .)