Do nothing with smelt from mild steel (4)

I believe the answer is:

idle

I'm a little stuck... Click here to teach me more about this clue!

'do nothing' is the definition.
(thesaurus)

I don't understand the rest of the clue.

(Other definitions for idle that I've seen before include "Loaf" , "Lied about being lazy or not working" , "Without work" , "Inactive - lazy" , "without purpose" .)

I've seen this clue in the Irish Times (Crosaire) and the Irish Times.

Want a hint initially instead of a full solution? Install my app