Fun involved a teen's mum (9)
I believe the answer is:
amusement
'fun' is the definition.
(amusement is fun or enjoyment)
'involved a teen's mum' is the wordplay.
'involved' indicates an anagram (involved can mean confusing or complex).
'a'+'teens'+'mum'='ateensmum'
'ateensmum' with letters rearranged gives 'AMUSEMENT'.
(Other definitions for amusement that I've seen before include "Light-hearted pastime" , "Idle wasting of time" , "set up through arcade?" , "Fun" , "Diversion, recreation" .)