Join in the fun, it entertains (5)
I believe the answer is:
unite
'join' is the definition.
(I know that join can be written as unite)
'in the fun it entertains' is the wordplay.
'in' says the answer is hidden in the clue.
'UNITE' is hidden in the letters of 'fun it entertains'.
(Other definitions for unite that I've seen before include "Join together, become allies" , "Put together" , "Join together, become one" , "Gather together as one" , "Get together" .)