Said “off key,” in a whisper (5)
I believe the answer is:
aside
'in a whisper' is the definition.
(thesaurus)
'said off key' is the wordplay.
'off' indicates an anagram.
'key' becomes 'e' (I've seen this in other clues**).
'said' with letters rearranged gives 'asid'.
'asid'+'e'='ASIDE'
(Other definitions for aside that I've seen before include "Line spoken to theatre audience" , "Quiet words spoken" , "Actor's covert line" , "Ideas (anag)" , "Actors comment to his audience" .)