Including it in the tax shows a lack of seriousness (6)
I believe the answer is:
levity
'a lack of seriousness' is the definition.
(I've seen this before)
'including it in the' is the wordplay.
'including' means one lot of letters goes inside another.
'in the' becomes 'levy' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'it' inserted into 'levy' is 'LEVITY'.
'tax shows' is the link.
This may not be right. Some or all of it may belong to another bit of the clue.
Can you help me to learn more?
(Other definitions for levity that I've seen before include "lack of respect" , "Lightness" , "Flippant attitude to serious matters" , "Facetiousness" , "Playfulness" .)