Most of cavity mother filled with one soft metal (7)

I believe the answer is:

holmium

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'soft metal' is the definition.
'holmium' can be an answer for 'metal' (holmium is a kind of metal). I am not certain of the 'soft' bit.

'most of cavity mother filled with one' is the wordplay.
'most of cavity' becomes 'hol' (I can't justify this - if you can you should believe this answer much more).
'mother' becomes 'mum' (informal term for mother).
'filled with' indicates putting letters inside.
'one' becomes 'i' (Roman numeral).
'mum' enclosing 'i' is 'mium'.
'hol'+'mium'='HOLMIUM'

Can you help me to learn more?

(Other definitions for holmium that I've seen before include "Metallic element" , "Chemical element with symbol Ho" .)

I've seen this clue in The Independent.

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