Taking a horse out unaccompanied, is nonsense! (7)
I believe the answer is:
baloney
'is nonsense' is the definition.
(I've seen this before)
'taking a horse out unaccompanied' is the wordplay.
'taking' indicates named letters should be taken away.
'horse' becomes 'bay' (bay is a kind of horse).
'out' means one lot of letters goes inside another.
'unaccompanied' becomes 'alone' (synonyms).
'bay' with 'a' taken away 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" .)