Where horse manure may go, confidentially (5,3,4)
I believe the answer is:
under the rose
'confidentially' is the definition.
(under the rose means in secret)
'where horse manure may go' is the wordplay.
I cannot quite see how this works, but
an anagram of 'horse' is 'heros' which is located in the answer.
This may be the basis of clue (or it may be nonsense).
(Other definitions for under the rose that I've seen before include "In confidence" , "Privately" , "where you might see mulch" , "in secret" , "Where Lancastrians, say, gathered" .)