Piece of metal that attracts iron (6)
I believe the answer is:
magnet
'piece of metal that attracts iron' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for magnet that I've seen before include "Source of attraction" , "Great attraction" , "Attractive piece of metal" , "Lodestone, for example" , "Centre of attraction" .)