Metal one worn by mother after disheartening operations (6)
I believe the answer is:
osmium
'metal' is the definition.
(osmium is a kind of metal)
'one worn by mother after disheartening operations' is the wordplay.
'one' becomes 'i' (Roman numeral).
'worn by' means one lot of letters goes inside another (I've seen this in other clues).
'mother' becomes 'mum' (mum is a kind of mother**).
'after' says to put letters next to each other.
'disheartening' suggests removing the centre.
'operations' with its middle removed is 'os'.
'i' going into 'mum' is 'mium'.
'mium' after 'os' is 'OSMIUM'.
(Other definitions for osmium that I've seen before include "Lustrous, extremely hard member of the platinum metals group" , "Densest of all naturally occurring metallic elements, Os" , "Metallic element" , "Silvery-white metal, Os" , "Metal, the densest natural element (symbol Os)" .)