Further remark about two riders initially on horseback? (7)
I believe the answer is:
astride
'on horseback?' is the definition.
(I've seen this in another clue)
'further remark about two riders initially' is the wordplay.
'further remark' becomes 'aside' (I have seen 'Confidential remark' mean 'aside' so perhaps 'remark' could also mean 'aside'. I am not sure about the 'further' bit.).
'about' means one lot of letters goes inside another.
'initially' indicates taking the first letters.
The initial letters of 'two riders' is 'tr'.
'aside' placed around 'tr' is 'ASTRIDE'.
Can you help me to learn more?
(Other definitions for astride that I've seen before include "It dares to be with legs wider apart" , "On the back of" , "With legs apart, emitting tirades" , "With feet apart" , "on horseback" .)