Slip again on horse (4)
I believe the answer is:
bish
'slip' is the definition.
(bish can mean an error)
'again on horse' is the wordplay.
'again' becomes 'bis' (musical term).
'on' is a charade indicator (letters next to each other) (in a down clue, letters appear on others).
'horse' becomes 'H' (abbreviation as in horse power).
'bis'+'h'='BISH'
(Other definitions for bish that I've seen before include "Blunder" , "Common mistake" .)