Grade scrap metal inside (6)
I believe the answer is:
'grade' is the definition.
(I know that grade can be written as rating)
'scrap metal inside' is the wordplay.
'scrap' becomes 'rag' (I've seen this before).
'metal' becomes 'tin' (tin is a kind of metal).
'inside' indicates putting letters inside.
'rag' enclosing 'tin' is 'RATING'.
(Other definitions for rating that I've seen before include "General classification or naval NCO" , "Score" , "rebuke?" , "Non-commissioned sailor in the Royal Navy" , "Sailor - assessment" .)