Grade scrap metal inside (6)
I believe the answer is:
rating
'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" .)