Negotiated hard deal about riding horse (7)
I believe the answer is:
haggled
'negotiated' is the definition.
(negotiated a price)
'hard deal about riding horse' is the wordplay.
'hard' becomes 'h' (abbreviation used in pencil classifications).
'about' indicates anagramming the letters.
'riding' indicates putting letters inside (some letters sit around or 'ride' others).
'horse' becomes 'gg' (geegee or GG is a childish term for a horse).
'deal' is an anagram of 'aled'.
'aled' going around 'gg' is 'aggled'.
'h'+'aggled'='HAGGLED'
(Other definitions for haggled that I've seen before include "Wrangled, over a price say" , "Argued about price or terms (7)" , "Bargained over price" .)