Hi Ripper
I was in a similar position. If you have the 'racehorse' you must have the five letters that have been taken out the wordplay of the numbered clues. So you need to work those back in to the grid entries. e.g. 1A definition is "Killer whale" but wordplay signifies "gam pu" i.e. "excited organ" reversed and you need to add, in this case, two of the five letters to get to the definition.
The unnumbered clues are normal and split into one group of eight that yields anagrams associated with five of the six "horses". The other five give shortened names of 'horses' that are contained within the solutions but omitted from the final grid entries. Hope that makes sense !