Crossword Genius

Jumping off top in fountainhead (8,5)

Ross

I believe the answer is:

starting point

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'fountainhead' is the definition.
Although both the answer and definition are singular nouns, I can't see how they can define each other.

'jumping off top in' is the wordplay.
'jumping' becomes 'starting' ('start' can be a synonym of 'jump').
'off' is an anagram indicator.
'top'+'in'='topin'
'topin' anagrammed gives 'point'.
'starting'+'point'='STARTING-POINT'

Can you help me to learn more?

(Other definitions for starting point that I've seen before include "The place from which one begins a journey" , "birth" , "Beginning" , "base" .)

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