Dark horse, frightful prospect (9)
I believe the answer is:
nightmare
'frightful prospect' is the definition.
The definition and answer can be both related to thinking as well as being singular nouns.
Maybe there's a link between them I don't understand?
'dark horse' is the wordplay.
'dark' becomes 'night' (it's dark at nighttime).
'horse' becomes 'mare' (mare is a kind of horse).
'night'+'mare'='NIGHTMARE'
Can you help me to learn more?
(Other definitions for nightmare that I've seen before include "Horror" , "causing distress to the Lord Chancellor?" , "that might upset the rest" , "It makes one restless" , "Scary fantasy while asleep" .)