One leap across South River (6)
I believe the answer is:
jester
'one' is the definition.
Although both the answer and definition are singular nouns, I don't see how they can define each other.
'leap across south river' is the wordplay.
'leap' becomes 'jete' (I can't justify this - if you can you should give a lot more credence to this answer).
'across' means one lot of letters goes inside another.
'south' becomes 's' (abbreviation).
'river' becomes 'r' (abbreviation).
'jete' going around 's' is 'jeste'.
'jeste'+'r'='JESTER'
Can you help me to learn more?
(Other definitions for jester that I've seen before include "Royal comedian" , "Court fool" , "Court humorist" , "Funny man, once at court" , "Comic" .)