Here are the children - Susan is first (5)
I believe the answer is:
issue
'here are the children' is the definition.
Although both the answer and definition are singular nouns, I can't see how one could define the other.
'susan is first' is the wordplay.
'susan' becomes 'sue' (short for Susan).
'first' says to put letters next to each other.
'sue' put after 'is' is 'ISSUE'.
Can you help me to learn more?
(Other definitions for issue that I've seen before include "one's children?" , "Come forth or bring out" , "Number (of periodical)" , "Contentious point" , "Copy of a magazine, for instance" .)