Summary given, suggests first, she brought horse up to sibling (8)
I believe the answer is:
synopsis
'summary' is the definition.
(I know that synopsis is a type of summary)
'first she brought horse up to sibling' is the wordplay.
'first she' becomes 's' (pst letter of 'she').
'brought horse' becomes 'pony' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'up' says the letters should be written backwards (in down clue: letters go upwards).
'to sibling' becomes 'sis' (short for sister).
'pony' back-to-front is 'ynop'.
's'+'ynop'+'sis'='SYNOPSIS'
'given suggests' acts as a link.
I am not very happy about this link. Some or all of it may belong to another bit of the clue.
Can you help me to learn more?
(Other definitions for synopsis that I've seen before include "Pr?cis, abstract" , "sketch" , "rundown?" , "Precis or summary" , "General summary" .)