Outline is horse climbing in sunless void (8)
I believe the answer is:
synopsis
'outline' is the definition.
(eg an outline of a story)
'is horse climbing in sunless void' is the wordplay.
'horse' becomes 'pony' (pony is a kind of horse).
'climbing' says the letters should be written in reverse (in a down clue, letters climb up).
'in' indicates putting letters inside.
'void' means to remove the middle letters (the word is empty or void of its inside letters).
'sunless' with its middle taken out is 'ss'.
'is'+'pony'='ispony'
'ispony' reversed gives 'ynopsi'.
'ynopsi' inserted inside 'ss' is 'SYNOPSIS'.
(Other definitions for synopsis that I've seen before include "rundown?" , "sketch" , "Precis or summary" , "General summary" , "Summary - brief outline" .)