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' suggests deleting specific letters (take can mean remove or delete).
'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' placed 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" .)