Jumping off top in fountainhead (8,5)
I believe the answer is:
starting point
'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" .)