Steel badly forged -- after this it shouldn't be purchased (4-2,4)
I believe the answer is:
sell-by date
'after this it shouldn't be purchased' is the definition.
(something should be sold by its sell-by date)
'steel badly forged' is the wordplay.
'forged' is an anagram indicator.
'steel'+'badly'='steelbadly'
'steelbadly' with letters rearranged gives 'SELL-BY-DATE'.
(Other definitions for sell-by date that I've seen before include "Food shop marking" , "Perishable deadline" , "Mark on a perishable" .)