Metal core moulded round a very soft centre (6)
I believe the answer is:
copper
'metal' is the definition.
(I know that copper is a type of metal)
'core moulded round a very soft centre' is the wordplay.
'moulded' indicates anagramming the letters.
'round' indicates putting letters inside.
'a very soft centre' becomes 'pp' (I can't explain this - if you can you should give a lot more credence to this answer).
'core' is an anagram of 'coer'.
'coer' placed around 'pp' is 'COPPER'.
Can you help me to learn more?
(Other definitions for copper that I've seen before include "Metal, Cu" , "Atomic number 29" , "A little cash" , "One who nicks" , "detector?" .)