Plot involving a legendary archer’s sidekick (9)
I believe the answer is:
satellite
'sidekick' is the definition.
(I've seen this before)
'plot involving a legendary archer's' is the wordplay.
'plot' becomes 'site' (as in a plot of land).
'involving' indicates putting letters inside.
'legendary archer' becomes 'Tell' (folk hero William Tell).
'a'+'tell'='atell'
'site' placed around 'atell' is 'SATELLITE'.
(Other definitions for satellite that I've seen before include "Moon" , "important person's attendant" , "Celestial body orbiting around another" , "dependent community" , "Body in orbit around another" .)