It's fun, mate (5)
I believe the answer is:
sport
This might be a double definition.
'it's fun' is the first definition.
(synonyms)
'mate' is the second definition.
The definition and answer can be both people as well as being singular nouns.
Perhaps there's an association between them I don't understand?
Can you help me to learn more?
(Other definitions for sport that I've seen before include "Eg, cricket" , "Physical exertion and competition" , "form of exercise?" , "Recreation, play" , "Abnormal" .)