It's bloody good source of metal (4)
I believe the answer is:
gore
'it's bloody' is the definition.
I can't judge whether this definition defines the answer.
'good source of metal' is the wordplay.
'source' suggests taking the first letters (source can mean a beginning or origin).
'of' becomes 'o''.
'metal' becomes 're' (Re is an example).
The initial letter of 'good' is 'g'.
'g'+'o'+'re'='GORE'
Can you help me to learn more?
(Other definitions for gore that I've seen before include "stuff from part of Kensington" , "Blood that's been shed" , "piece of material" , "Coagulated blood from a wound" , "Pierce or wound with a horn or tusk" .)