"It's — a pleasure” (4)
'it's a pleasure' is the definition.
(fun is enjoyment or pleasure)
This is all the clue.
(Other definitions for fun that I've seen before include "Light-hearted amusement, jollity" , "Playful amusement" , "Light-hearted pleasure" , "Jollification" , "Entertaining" .)
(Other definitions for been that I've seen before include "(Have) gone in the past (to somewhere)" , "Past participle of be" .)