Slaves sneaked up after a horse (6)
I believe the answer is:
helots
'slaves' is the definition.
(I've seen this in another clue)
'sneaked up after a horse' is the wordplay.
'sneaked' becomes 'stole' ('steal' can be a synonym of 'sneak').
'up' says the letters should be written backwards (in down clue: letters go upwards).
'after' means one lot of letters go next to another.
'a horse' becomes 'H' (abbreviation as in horse power).
'stole' backwards is 'elots'.
'elots' after 'h' is 'HELOTS'.
(Other definitions for helots that I've seen before include "Serfs" , "Ancient Spartans, in hotels?" , "Slave labour" , "They worked like slaves" .)