Small sibling holding up small horse, in summary (8)
I believe the answer is:
synopsis
'summary' is the definition.
(I know that synopsis is a type of summary)
'small sibling holding up small horse' is the wordplay.
'small' becomes 's' (abbreviation - e.g. clothes size).
'sibling' becomes 'sis' (short for sister).
'holding' indicates putting letters inside.
'up' says the letters should be written backwards (in down clue: letters go upwards).
'small horse' becomes 'pony' (I've seen this before).
'pony' reversed gives 'ynop'.
's'+'sis'='ssis'
'ssis' going around 'ynop' is 'SYNOPSIS'.
'in' is the link.
(Other definitions for synopsis that I've seen before include "sketch" , "Precis or summary" , "rundown?" , "Summary - brief outline" , "Pr?cis, abstract" .)