A good-hearted bit of fun (4)
I believe the answer is:
joke
This might be a double definition.
'a good-hearted bit' is the first definition.
'joke' can be an answer for 'bit' (I have seen 'bit of humour' mean 'joke' so perhaps 'bit' could also mean 'joke'). I'm not sure about the 'a good-hearted' bit.
'fun' is the second definition.
(thesaurus)
'of' acts as a link.
Can you help me to learn more?
(Other definitions for joke that I've seen before include "Witticism, jest" , "Humorous remark" , "Funny story" , "someone inadequate" , "Quip, jest" .)