Disconcert group of persons on horseback? (5)
I believe the answer is:
upset
'disconcert' is the definition.
(synonyms)
'group of persons on horseback?' is the wordplay.
I cannot really see how this works, but
'group' could be 'set' (both can mean a collection) and 'set' is located in the answer.
'on horseback?' could be 'up' (up can mean up on a horse) and 'up' is found in the answer.
No letters remain.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for upset that I've seen before include "Disturb, physically or emotionally" , "Make unhappy; knock over" , "Overturn; distress" , "overcome" , "Unexpected defeat - tipped over" .)