Smooth metallic element, widely used (4)

I believe the answer is:

iron

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'smooth' is the definition.
(to smooth down with an iron)

'metallic element widely used' is the wordplay.
'metallic element' becomes 'Ir' (Ir is an example).
'widely used' becomes 'on' (I can't explain this - if you can you should believe this answer much more).
'ir'+'on'='IRON'

Can you help me to learn more?

(Other definitions for iron that I've seen before include "Smooth - driver?" , "Strong metal; golf club" , "Press clothes" , "Smoother - metal" , "Metal - press" .)

I've seen this clue in The Irish Times.

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