Source of HG's soft call (10)

I believe the answer is:

wellspring

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'source' is the definition.
(I know that wellspring is a type of source)

'hg's soft call' is the wordplay.
'hg' becomes 'wells' (I can't justify this - if you can you should believe this answer much more).
'soft' becomes 'p' (musical abbreviation for 'piano', soft).
'call' becomes 'ring' (telephone call).
'wells'+'p'+'ring'='WELLSPRING'

'of' acts as a link.

Can you help me to learn more?

(Other definitions for wellspring that I've seen before include "Plentiful supply" , "Abundant source" , "Rich provider" .)

I've seen this clue in the Sydney Morning Herald.

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