Join in the fun, it entertains (5)
I believe the answer is:
unite
'join' is the definition.
(synonyms)
'in the fun it entertains' is the wordplay.
'in' indicates a hidden word.
'UNITE' is hidden within '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" .)