Fun with a "mate" (5)
I believe the answer is:
sport
'fun' is the definition.
(I know that fun can be written as sport)
I can't explain the rest of the clue.
Can you help me to learn more?
(Other definitions for sport that I've seen before include "Exhibit publicly; eg soccer" , "Eg, football; fun" , "Recreation, play" , "Physical recreation" , "Wear, display (an item)" .)