Thoughtlessness of tax involving computer services (6)
I believe the answer is:
levity
'thoughtlessness' is the definition.
(I've seen this before)
'tax involving computer services' is the wordplay.
'tax' becomes 'levy' (I've seen this before).
'involving' indicates putting letters inside.
'computer services' becomes 'it' (abbreviation for Information Technology).
'levy' enclosing 'it' is 'LEVITY'.
'of' is the link.
(Other definitions for levity that I've seen before include "Lightness" , "Disrespectful treatment of a serious matter" , "Unseriousness" , "Facetiousness" , "Playfulness" .)