Son is into black example of humour (4)
I believe the answer is:
jest
'example of humour' is the definition.
(a jest is a joke or teasing comment)
'son is into black' is the wordplay.
'son' becomes 's' (genealogical abbreviation for son).
'is into' is an insertion indicator.
'black' becomes 'jet' (similar in meaning).
's' placed inside 'jet' is 'JEST'.
(Other definitions for jest that I've seen before include "In ____, playfully" , "Scoff; playfulness" , "Humorous remark" , "Banter, joke" , "fun" .)