They follow the horses and are number ten in the charts (8)
I believe the answer is:
chariots
'they follow the horses and' is the definition.
The definition suggests an adverb but the answer is not.
'number ten in the charts' is the wordplay.
'number ten' becomes 'IO' (IO resembles 10).
'in the' means one lot of letters goes inside another.
'io' inserted inside 'charts' is 'CHARIOTS'.
'are' is the link.
Can you help me to learn more?
(Other definitions for chariots that I've seen before include "Ancient racing vehicles" , "Things once drawn" , "Film about the 1924 Olympics" , "Horse-drawn carriages once used in battle as by Romans" , "Ancient horse-drawn vehicles" .)