Molten core then sticking together (8)
I believe the answer is:
coherent
'sticking together' is the definition.
(I've seen this before)
'molten core then' is the wordplay.
'molten' is an anagram indicator (I've seen this in other clues).
'core'+'then'='corethen'
'corethen' is an anagram of 'COHERENT'.
(Other definitions for coherent that I've seen before include "Sticking together" , "Rational" , "Holding together as a whole" , "Able to speak clearly" , "Not rambling in speech or reasoning" .)