She’s optimistic about newly-trained mare (8)
I believe the answer is:
rosemary
'she's' is the definition.
(I've seen this before)
'optimistic about newly-trained mare' is the wordplay.
'optimistic' becomes 'rosy' ('rosy' can be similar in meaning to 'optimistic').
'about' means one lot of letters goes inside another.
'newly trained' is an anagram indicator.
'mare' is an anagram of 'emar'.
'rosy' going around 'emar' is 'ROSEMARY'.
(Other definitions for rosemary that I've seen before include "Fragrant evergreen shrub and herb" , "Plant" , "Culinary herb" , "Helps one to remember" , "-'s Baby, Roman Polanski film" .)