Competition is a game of golf with hero's sidekick (5,5)
I believe the answer is:
round robin
'competition' is the definition.
(I know that round robin is a type of tournament)
'game of golf with hero's sidekick' is the wordplay.
'game of golf' becomes 'round' (I've seen this before).
'with' is a charade indicator (letters next to each other).
'hero's sidekick' becomes 'robin' (I can't explain this - if you can you should believe this answer much more).
'round'+'robin'='ROUND ROBIN'
'is a' acts as a link.
Can you help me to learn more?
(Other definitions for round robin that I've seen before include "Circular letter, petition" , "note" , "Statement signed" , "Tournament in which every contestant plays against every other" , "to make protest" .)