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

I believe the answer is:

holmium

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

'soft metal' is the definition.
'holmium' can be an answer for 'metal' (holmium is a kind of metal). I'm unsure of the 'soft' bit.

'most of cavity mother filled with one' is the wordplay.
'most of cavity' becomes 'hol' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'mother' becomes 'mum' (informal term for mother).
'filled with' is an insertion indicator.
'one' becomes 'i' (Roman numeral).
'mum' going around '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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