Moulded mass of tin, possibly, containing some gold (5)
I believe the answer is:
ingot
'some gold' is the definition.
(thesaurus)
'moulded mass of tin possibly containing' is the wordplay.
'moulded mass' becomes 'g' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'of' becomes 'o''.
'possibly' is an anagram indicator.
'containing' indicates putting letters inside.
'tin' anagrammed gives 'int'.
'g'+'o'='go'
'go' inserted within 'int' is 'INGOT'.
Can you help me to learn more?
(Other definitions for ingot that I've seen before include "Bar, often gold" , "Block of cast metal" , "Metal casting" , "expensive block" , "Block (of steel, gold, etc)" .)