One maybe developing a plot, reportedly for Hollywood star (7)
I believe the answer is:
gardner
'one' is the definition.
Both the answer and definition are singular nouns.
Maybe you can see a link between them that I don't see?
I don't understand how the rest of the clue works.
Can you help me to learn more?
(Other definitions for gardner that I've seen before include "Writer of detective books" , "Erle Stanley ..., Ava ..." , "Ava --, US actress" , "Ava and Earl Stanley" , "Ava ..., glamorous old film star" .)