Outline of first sketch by sibling : “Horse climbing” (8)
I believe the answer is:
synopsis
'outline' is the definition.
(synonyms)
'first sketch by sibling horse climbing' is the wordplay.
'first sketch' becomes 's' (pst letter of 'sketch').
'by' says to put letters next to each other.
'sibling' becomes 'sis' (I have seen 'Little sibling ' mean 'sis' so perhaps 'sibling' could also mean 'sis').
'horse' becomes 'pony' (pony is a kind of horse).
'climbing' is a reversal indicator (in a down clue, letters climb up).
'sis'+'pony'='sispony'
'sispony' backwards is 'ynopsis'.
's'+'ynopsis'='SYNOPSIS'
'of' is the link.
(Other definitions for synopsis that I've seen before include "sketch" , "Summary - brief outline" , "General summary" , "Pr?cis, abstract" , "rundown?" .)