Thanks, Gitto. I wondered why you'd mentioned 'steed' - now I know! The American steed has four letters, the first is the dropped one in the entry, then add 'start to ache', in crossword terms.
All finished, but not seeing the whole message from the across extras.
As has been said, not essential.
In 19a and 21a, I can't see the extra letter. But I get the gist!