To include it in the tax shows flippancy (6)
I believe the answer is:
levity
'flippancy' is the definition.
(I've seen this in another clue)
'to include it in the' is the wordplay.
'to include' is an insertion indicator.
'in the' becomes 'levy' (I can't explain this - if you can you should give a lot more credence to this answer).
'it' put into 'levy' is 'LEVITY'.
'tax shows' is the link.
I am not very happy about this link. Some or all of it may be part of another bit of the clue.
Can you help me to learn more?
(Other definitions for levity that I've seen before include "Lightness" , "Lack of seriousness" , "Light-heartedness" , "Frivolous tendency" , "Facetiousness" .)