It's fun, cobber (5)
I believe the answer is:
sport
This clue could be a double definition.
'it's fun' is the first definition.
(synonyms)
'cobber' is the second definition.
The answer and definition can be both people as well as being singular nouns.
Maybe you can see an association between them that I don't see?
Can you help me to learn more?
(Other definitions for sport that I've seen before include "Physical activity" , "Games" , "Wear - game - generous type" , "Physical exertion and competition" , "Put on" .)