Sort of iron that's approximate in weight (7)
I believe the answer is:
wrought
'sort of iron' is the definition.
'approximate in weight' is the wordplay.
'approximate' becomes 'rough' (I've seen this in another clue).
'in' indicates putting letters inside.
'weight' becomes 'wt'.
'rough' going into 'wt' is 'WROUGHT'.
'that's' is the link.
(Other definitions for wrought that I've seen before include "knocked into shape" , "Made" , "Ornamented" , "Shaped by hammering" , "Worked by hammering or beating" .)