They reveal embarrassment having left out these former Presidents (6)
I believe the answer is:
bushes
'they' is the definition.
Both the answer and definition are plural nouns.
Maybe you can see an association between them that I don't see?
I cannot understand how the remainder of the clue works.
Can you help me to learn more?
(Other definitions for bushes that I've seen before include "Low woody plants" , "Shrubs" .)