In one aspect, it's simply not a stage whisper (5)
I believe the answer is:
aside
'a stage whisper' is the definition.
(both can mean a remark heard by an audience)
'in one aspect' is the wordplay.
'in one' becomes 'a' (a thing is one thing. I am not sure about the 'in' bit.).
'aspect' becomes 'side' (side is a kind of aspect).
'a'+'side'='ASIDE'
'it's simply not' is the link.
This may not be right. Some or all of it may belong to another bit of the clue.
Can you help me to learn more?
(Other definitions for aside that I've seen before include "Apart; incidental remark" , "Actor's remark for the audience alone" , "Separately" , "Digression" , "Actor's covert line" .)