Taking a horse out unaccompanied, is nonsense! (7)
I believe the answer is:
baloney
'is nonsense' is the definition.
(I've seen this in another clue)
'taking a horse out unaccompanied' is the wordplay.
'taking' is a deletion indicator (take can mean remove or delete).
'horse' becomes 'bay' (bay is a kind of horse).
'out' indicates putting letters inside.
'unaccompanied' becomes 'alone' (synonyms).
'bay' with 'a' taken out is 'by'.
'by' going around 'alone' is 'BALONEY'.
(Other definitions for baloney that I've seen before include "rhubarb" , "foolish talk!" , "Tripe < , smoked sausage" , "Nonsense; lean yob (anag)" , "Tosh" .)