An actor's "aside"? (5)
I believe the answer is:
apart
'aside?' is the definition.
('apart' can be a synonym of 'aside')
'an actor's' is the wordplay.
'an' becomes 'a'.
'actor' becomes 'part' (I can't justify this - if you can you should believe this answer much more).
'a'+'part'='APART'
Can you help me to learn more?
(Other definitions for apart that I've seen before include "(Break) into pieces" , "in bits" , "divorced" , "Separated; to one side" , "cut off from others" .)