Extraterrestrial's son's declining wits (6)
I believe the answer is:
comics
'wits' is the definition.
(wit can mean a comical person)
I cannot understand how the remainder of the clue works.
(Other definitions for comics that I've seen before include "Funny performers" , "Cartoon periodicals" , "funny material" , "Children's periodicals" , "Comedians" .)